add expected warning message.

Author: valeriy42

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

@@ -110,7 +110,12 @@ setup:
 
 ---
 "Test flush and close job WITHOUT sending any data":
+  - skip:
+      features:
+        - "warnings"
   - do:
+      warnings:
+        - 'Forcing any buffered data to be processed is deprecated, in a future major version it will be compulsory to use a datafeed'
       ml.flush_job:
         job_id: post-data-job
   - match: { flushed: true }