Wait for green in mlt tests

rest-api-spec/test/mlt/10_basic.yaml

@@ -10,6 +10,10 @@
   - do:
       indices.refresh: {}
 
+  - do:
+      cluster.health:
+           wait_for_status: green
+           timeout: 1s   
   - do:
       mlt:
           index:  test_1