Fixing flakiness of ShuffleForcedMergePolicyTests (#3591) (#3592)

Signed-off-by: Andriy Redko <andriy.redko@aiven.io>
(cherry picked from commit 055a9fc)

Co-authored-by: Andriy Redko <andriy.redko@aiven.io>

server/src/test/java/org/opensearch/lucene/index/ShuffleForcedMergePolicyTests.java

@@ -58,7 +58,7 @@
 public class ShuffleForcedMergePolicyTests extends BaseMergePolicyTestCase {
     public void testDiagnostics() throws IOException {
         try (Directory dir = newDirectory()) {
-            IndexWriterConfig iwc = newIndexWriterConfig();
+            IndexWriterConfig iwc = newIndexWriterConfig().setMaxFullFlushMergeWaitMillis(0);
             MergePolicy mp = new ShuffleForcedMergePolicy(newTieredMergePolicy());
             iwc.setMergePolicy(mp);
             boolean sorted = random().nextBoolean();