Fix YAML test expression parser

Author: miguelgrinberg

test_elasticsearch/test_server/test_rest_api_spec.py

@@ -432,11 +432,17 @@ def _lookup(self, path):
                 step = int(step)
                 assert isinstance(value, list)
                 assert len(value) > step
+                value = value[step]
             elif step == "_arbitrary_key_":
                 return list(value.keys())[0]
-            else:
+            elif (
+                isinstance(step, string_types)
+                and isinstance(value, dict)
+            ):
                 assert step in value
-            value = value[step]
+                value = value[step]
+            else:
+                value = step
         return value
 
     def _feature_enabled(self, name):