[ML] wait for yellow state for stats index in tests (#58436) (#58456)

GET inference stats now reads from the .ml-stats index.

Our tests should wait for yellow state before attempting to query the index for stat information.

x-pack/plugin/src/test/resources/rest-api-spec/test/ml/inference_stats_crud.yml

@@ -96,6 +96,12 @@ setup:
               }
             ]
           }
+  - do:
+      headers:
+        Authorization: "Basic eF9wYWNrX3Jlc3RfdXNlcjp4LXBhY2stdGVzdC1wYXNzd29yZA==" # run as x_pack_rest_user, i.e. the test setup superuser
+      cluster.health:
+        wait_for_status: yellow
+        index: .ml-stats*
 ---
 "Test get stats given missing trained model":