Another test fix

Another attempt to fix a test that fails rarely and randomly. This time
try locking the query to just a single index.

Author: nik9000

x-pack/plugin/src/test/resources/rest-api-spec/test/analytics/top_metrics.yml

@@ -174,6 +174,7 @@
   - do:
       catch: bad_request
       search:
+        index: test
         size: 0
         body:
           aggs:
@@ -269,6 +270,7 @@
       catch: bad_request
       search:
         size: 0
+        index: test
         body:
           aggs:
             tm: