Make test setup more reliable elastic#121110 (elastic#121313)

Make test setup more reliable
Resolves elastic#119197

(cherry picked from commit 748801e)

Author: svilen-mihaylov-elastic

rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/search.vectors/42_knn_search_int4_flat.yml

@@ -62,6 +62,17 @@ setup:
   - do:
       indices.refresh: {}
 
+  # For added test reliability, pending the resolution of https://github.com/elastic/elasticsearch/issues/109416.
+  - do:
+      indices.forcemerge:
+        max_num_segments: 1
+        index: int4_flat
+  - do:
+      indices.refresh: {}
+  - do:
+      indices.forcemerge:
+        max_num_segments: 1
+        index: int4_flat
 ---
 "kNN search only":
   - do: