[TEST] Wait for merging to complete before testing breaker

It's possible that a merge may be ongoing when we check the breaker and segment
stats' memory usage, this causes the test to fail. Instead, we should wait for
merging to complete.

Resolves #27651

core/src/test/java/org/elasticsearch/index/shard/IndexShardTests.java

@@ -2854,6 +2854,10 @@ public class IndexShardTests extends IndexShardTestCase {
             t.join();
         }
 
+        // We need to wait for all ongoing merges to complete. The reason is that during a merge the
+        // IndexWriter holds the core cache key open and causes the memory to be registered in the breaker
+        primary.forceMerge(new ForceMergeRequest());
+
         // Close remaining searchers
         IOUtils.close(searchers);