Ensure force merge to single segment is flushed (#5652)

Signed-off-by: Daniel Widdis <widdis@gmail.com>
(cherry picked from commit 30ac284)
Signed-off-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>

rest-api-spec/src/main/resources/rest-api-spec/test/search.aggregation/20_terms.yml

@@ -824,6 +824,7 @@ setup:
       indices.forcemerge:
         index: test_1
         max_num_segments: 1
+        flush: true
 
   - do:
       search: