Added tests for GET /_mapping/field/{fields}.

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

tests/default/_core/mapping/field.yaml

@@ -0,0 +1,28 @@
+$schema: ../../../../json_schemas/test_story.schema.yaml
+
+description: Test field mappings.
+prologues:
+  - path: /{index}
+    method: PUT
+    parameters:
+      index: movies
+    request:
+      payload:
+        mappings:
+          properties:
+            director:
+              type: text
+            year:
+              type: integer
+epilogues:
+  - path: /movies
+    method: DELETE
+    status: [200, 404]
+chapters:
+  - synopsis: Get field mappings across multiple indexes.
+    path: /_mapping/field/{fields}
+    method: GET
+    parameters:
+      fields: year
+    response:
+      status: 200