[SPARK-20410][SQL] Make sparkConf a def in SharedSQLContext

## What changes were proposed in this pull request?
It is kind of annoying that `SharedSQLContext.sparkConf` is a val when overriding test cases, because you cannot call `super` on it. This PR makes it a function.

## How was this patch tested?
Existing tests.

Author: Herman van Hovell <hvanhovell@databricks.com>

Closes #17705 from hvanhovell/SPARK-20410.

sql/core/src/test/scala/org/apache/spark/sql/AggregateHashMapSuite.scala

@@ -19,13 +19,12 @@ package org.apache.spark.sql
 
 import org.scalatest.BeforeAndAfter
 
-class SingleLevelAggregateHashMapSuite extends DataFrameAggregateSuite with BeforeAndAfter {
+import org.apache.spark.SparkConf
 
-  protected override def beforeAll(): Unit = {
-    sparkConf.set("spark.sql.codegen.fallback", "false")
-    sparkConf.set("spark.sql.codegen.aggregate.map.twolevel.enable", "false")
-    super.beforeAll()
-  }
+class SingleLevelAggregateHashMapSuite extends DataFrameAggregateSuite with BeforeAndAfter {
+  override protected def sparkConf: SparkConf = super.sparkConf
+    .set("spark.sql.codegen.fallback", "false")
+    .set("spark.sql.codegen.aggregate.map.twolevel.enable", "false")
 
   // adding some checking after each test is run, assuring that the configs are not changed
   // in test code
@@ -38,12 +37,9 @@ class SingleLevelAggregateHashMapSuite extends DataFrameAggregateSuite with Befo
 }
 
 class TwoLevelAggregateHashMapSuite extends DataFrameAggregateSuite with BeforeAndAfter {
-
-  protected override def beforeAll(): Unit = {
-    sparkConf.set("spark.sql.codegen.fallback", "false")
-    sparkConf.set("spark.sql.codegen.aggregate.map.twolevel.enable", "true")
-    super.beforeAll()
-  }
+  override protected def sparkConf: SparkConf = super.sparkConf
+    .set("spark.sql.codegen.fallback", "false")
+    .set("spark.sql.codegen.aggregate.map.twolevel.enable", "true")
 
   // adding some checking after each test is run, assuring that the configs are not changed
   // in test code
@@ -55,15 +51,14 @@ class TwoLevelAggregateHashMapSuite extends DataFrameAggregateSuite with BeforeA
   }
 }
 
-class TwoLevelAggregateHashMapWithVectorizedMapSuite extends DataFrameAggregateSuite with
-BeforeAndAfter {
+class TwoLevelAggregateHashMapWithVectorizedMapSuite
+  extends DataFrameAggregateSuite
+  with BeforeAndAfter {
 
-  protected override def beforeAll(): Unit = {
-    sparkConf.set("spark.sql.codegen.fallback", "false")
-    sparkConf.set("spark.sql.codegen.aggregate.map.twolevel.enable", "true")
-    sparkConf.set("spark.sql.codegen.aggregate.map.vectorized.enable", "true")
-    super.beforeAll()
-  }
+  override protected def sparkConf: SparkConf = super.sparkConf
+    .set("spark.sql.codegen.fallback", "false")
+    .set("spark.sql.codegen.aggregate.map.twolevel.enable", "true")
+    .set("spark.sql.codegen.aggregate.map.vectorized.enable", "true")
 
   // adding some checking after each test is run, assuring that the configs are not changed
   // in test code

sql/core/src/test/scala/org/apache/spark/sql/DatasetSerializerRegistratorSuite.scala

@@ -20,22 +20,20 @@ package org.apache.spark.sql
 import com.esotericsoftware.kryo.{Kryo, Serializer}
 import com.esotericsoftware.kryo.io.{Input, Output}
 
+import org.apache.spark.SparkConf
 import org.apache.spark.serializer.KryoRegistrator
 import org.apache.spark.sql.test.SharedSQLContext
-import org.apache.spark.sql.test.TestSparkSession
 
 /**
  * Test suite to test Kryo custom registrators.
  */
 class DatasetSerializerRegistratorSuite extends QueryTest with SharedSQLContext {
   import testImplicits._
 
-  /**
-   * Initialize the [[TestSparkSession]] with a [[KryoRegistrator]].
-   */
-  protected override def beforeAll(): Unit = {
-    sparkConf.set("spark.kryo.registrator", TestRegistrator().getClass.getCanonicalName)
-    super.beforeAll()
+
+  override protected def sparkConf: SparkConf = {
+    // Make sure we use the KryoRegistrator
+    super.sparkConf.set("spark.kryo.registrator", TestRegistrator().getClass.getCanonicalName)
   }
 
   test("Kryo registrator") {

sql/core/src/test/scala/org/apache/spark/sql/execution/DataSourceScanExecRedactionSuite.scala

@@ -18,22 +18,17 @@ package org.apache.spark.sql.execution
 
 import org.apache.hadoop.fs.Path
 
+import org.apache.spark.SparkConf
 import org.apache.spark.sql.QueryTest
 import org.apache.spark.sql.test.SharedSQLContext
-import org.apache.spark.util.Utils
 
 /**
  * Suite that tests the redaction of DataSourceScanExec
  */
 class DataSourceScanExecRedactionSuite extends QueryTest with SharedSQLContext {
 
-  import Utils._
-
-  override def beforeAll(): Unit = {
-    sparkConf.set("spark.redaction.string.regex",
-      "file:/[\\w_]+")
-    super.beforeAll()
-  }
+  override protected def sparkConf: SparkConf = super.sparkConf
+    .set("spark.redaction.string.regex", "file:/[\\w_]+")
 
   test("treeString is redacted") {
     withTempDir { dir =>

sql/core/src/test/scala/org/apache/spark/sql/execution/datasources/FileSourceStrategySuite.scala

@@ -42,7 +42,7 @@ import org.apache.spark.util.Utils
 class FileSourceStrategySuite extends QueryTest with SharedSQLContext with PredicateHelper {
   import testImplicits._
 
-  protected override val sparkConf = new SparkConf().set("spark.default.parallelism", "1")
+  protected override def sparkConf = super.sparkConf.set("spark.default.parallelism", "1")
 
   test("unpartitioned table, single partition") {
     val table =

sql/core/src/test/scala/org/apache/spark/sql/execution/streaming/CompactibleFileStreamLogSuite.scala

@@ -28,8 +28,8 @@ import org.apache.spark.sql.test.SharedSQLContext
 class CompactibleFileStreamLogSuite extends SparkFunSuite with SharedSQLContext {
 
   /** To avoid caching of FS objects */
-  override protected val sparkConf =
-    new SparkConf().set(s"spark.hadoop.fs.$scheme.impl.disable.cache", "true")
+  override protected def sparkConf =
+    super.sparkConf.set(s"spark.hadoop.fs.$scheme.impl.disable.cache", "true")
 
   import CompactibleFileStreamLog._
 

sql/core/src/test/scala/org/apache/spark/sql/execution/streaming/HDFSMetadataLogSuite.scala

@@ -38,8 +38,8 @@ import org.apache.spark.util.UninterruptibleThread
 class HDFSMetadataLogSuite extends SparkFunSuite with SharedSQLContext {
 
   /** To avoid caching of FS objects */
-  override protected val sparkConf =
-    new SparkConf().set(s"spark.hadoop.fs.$scheme.impl.disable.cache", "true")
+  override protected def sparkConf =
+    super.sparkConf.set(s"spark.hadoop.fs.$scheme.impl.disable.cache", "true")
 
   private implicit def toOption[A](a: A): Option[A] = Option(a)
 

sql/core/src/test/scala/org/apache/spark/sql/test/SharedSQLContext.scala

@@ -30,7 +30,9 @@ import org.apache.spark.sql.{SparkSession, SQLContext}
  */
 trait SharedSQLContext extends SQLTestUtils with BeforeAndAfterEach with Eventually {
 
-  protected val sparkConf = new SparkConf()
+  protected def sparkConf = {
+    new SparkConf().set("spark.hadoop.fs.file.impl", classOf[DebugFilesystem].getName)
+  }
 
   /**
    * The [[TestSparkSession]] to use for all tests in this suite.
@@ -51,8 +53,7 @@ trait SharedSQLContext extends SQLTestUtils with BeforeAndAfterEach with Eventua
   protected implicit def sqlContext: SQLContext = _spark.sqlContext
 
   protected def createSparkSession: TestSparkSession = {
-    new TestSparkSession(
-      sparkConf.set("spark.hadoop.fs.file.impl", classOf[DebugFilesystem].getName))
+    new TestSparkSession(sparkConf)
   }
 
   /**