[SPARK-17738][TEST] Fix flaky test in ColumnTypeSuite

## What changes were proposed in this pull request?

The default buffer size is not big enough for randomly generated MapType.

## How was this patch tested?

Ran the tests in 100 times, it never fail (it fail 8 times before the patch).

Author: Davies Liu <davies@databricks.com>

Closes #15395 from davies/flaky_map.

sql/core/src/test/scala/org/apache/spark/sql/execution/columnar/ColumnTypeSuite.scala

@@ -101,14 +101,15 @@ class ColumnTypeSuite extends SparkFunSuite with Logging {
 
   def testColumnType[JvmType](columnType: ColumnType[JvmType]): Unit = {
 
-    val buffer = ByteBuffer.allocate(DEFAULT_BUFFER_SIZE).order(ByteOrder.nativeOrder())
     val proj = UnsafeProjection.create(Array[DataType](columnType.dataType))
     val converter = CatalystTypeConverters.createToScalaConverter(columnType.dataType)
     val seq = (0 until 4).map(_ => proj(makeRandomRow(columnType)).copy())
+    val totalSize = seq.map(_.getSizeInBytes).sum
+    val bufferSize = Math.max(DEFAULT_BUFFER_SIZE, totalSize)
 
     test(s"$columnType append/extract") {
-      buffer.rewind()
-      seq.foreach(columnType.append(_, 0, buffer))
+      val buffer = ByteBuffer.allocate(bufferSize).order(ByteOrder.nativeOrder())
+      seq.foreach(r => columnType.append(columnType.getField(r, 0), buffer))
 
       buffer.rewind()
       seq.foreach { row =>