Added tests for GET and POST /{index}/_explain/{id}.

Signed-off-by: dblock <dblock@amazon.com>

spec/schemas/_core.explain.yaml

@@ -39,7 +39,7 @@ components:
           items:
             $ref: '#/components/schemas/ExplanationDetail'
         value:
-          oneOf:
+          anyOf:
             - type: integer
               format: int32
             - type: integer

tests/default/indices/explain.yaml

@@ -0,0 +1,51 @@
+$schema: ../../../json_schemas/test_story.schema.yaml
+
+description: Test getting an explanation of how the relevance score is calculated for every result.
+epilogues:
+  - path: /movies
+    method: DELETE
+    status: [200, 404]
+prologues:
+  - path: /_bulk
+    method: POST
+    parameters:
+      refresh: true
+    request:
+      content_type: application/x-ndjson
+      payload:
+        - {create: {_index: movies, _id: movie1}}
+        - {director: Bennett Miller, title: The Cruise, year: 1998}
+        - {create: {_index: movies, _id: movie2}}
+        - {director: Nicolas Winding Refn, title: Drive, year: 1960}
+chapters:
+  - synopsis: Explain the score for a match query (GET).
+    path: /{index}/_explain/{id}
+    method: GET
+    parameters:
+      index: movies
+      id: movie1
+    request:
+      payload:
+        query:
+          match:
+            year: 1998
+    response:
+      status: 200
+  - synopsis: Explain the score for a match query (POST).
+    path: /{index}/_explain/{id}
+    method: POST
+    parameters:
+      index: movies
+      id: movie2
+      preference: _local
+      routing: primary
+      _source: true
+      _source_excludes: title
+      _source_includes: director
+    request:
+      payload:
+        query:
+          match:
+            title: Drive
+    response:
+      status: 200