Added waiting for yellow cluster state in rest test

In order to prevent rare timing issue, where the creation
of an index happens in the cluster state too late and thus
results in empty mappings.

rest-api-spec/test/indices.get_field_mapping/50_field_wildcards.yaml

@@ -46,6 +46,10 @@ setup:
                           type:   string
                           index_name: t3
 
+  - do:
+      cluster.health:
+          wait_for_status: yellow
+
 ---
 "Get field mapping with * for fields":