Ensure green step in synonyms rule yaml test (elastic#114400)

Fixes test issue serverless 2922.

Author: kingherc

rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/synonyms/60_synonym_rule_get.yml

@@ -13,7 +13,10 @@ setup:
               id: "test-id-2"
             - synonyms: "test => check"
               id: "test-id-3"
-
+  - do:
+      cluster.health:
+        index: .synonyms-2
+        wait_for_status: green
 
 ---
 "Get a synonym rule":