Fix flaky test case - string profiler via global ordinals (opensearch-project#2226)

forcemerge to one segment before executing aggregation query.

Signed-off-by: Peng Huo <[email protected]>

Author: penghuo

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

@@ -820,6 +820,10 @@ setup:
           { "str": "cow", "number": 1 }
           { "index": {} }
           { "str": "pig", "number": 1 }
+  - do:
+      indices.forcemerge:
+        index: test_1
+        max_num_segments: 1
 
   - do:
       search: