[7.x] Implement 'contains' feature for REST API tests

Co-authored-by: P. Sai Vinay <[email protected]>

test_elasticsearch/test_server/test_rest_api_spec.py

@@ -50,6 +50,7 @@ IMPLEMENTED_FEATURES = {
     "default_shards",
     "warnings",
     "allowed_warnings",
+    "contains",
 }
 
 # broken YAML tests on some releases
@@ -276,6 +277,14 @@ class YamlRunner:
             else:
                 assert expected == value, "%r does not match %r" % (value, expected)
 
+    def run_contains(self, action):
+        for path, expected in action.items():
+            value = self._lookup(path)  # list[dict[str,str]] is returned
+            expected = self._resolve(expected)  # dict[str, str]
+
+            if expected not in value:
+                raise AssertionError("%s is not contained by %s" % (expected, value))
+
     def _resolve(self, value):
         # resolve variables
         if isinstance(value, string_types) and value.startswith("$"):