Ensure green step in synonyms rule yaml test (#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":