Fix testShouldPeriodicallyFlushAfterMerge (#52243)

MockRandomMergePolicy randomly determines if a segment should use a 
compound format. This can cause a force merge performing two merges: (1)
merging to a single segment, (2) rewriting the new segment using the
compound format. If the second merge completes after we have flushed,
then it can flip the flag shouldPeriodicallyFlushAfterBigMerge to true.

Closes #52205

Author: dnhatn

server/src/test/java/org/elasticsearch/index/engine/InternalEngineTests.java

@@ -4753,6 +4753,9 @@ public void testShouldPeriodicallyFlush() throws Exception {
     }
 
     public void testShouldPeriodicallyFlushAfterMerge() throws Exception {
+        engine.close();
+        // Do not use MockRandomMergePolicy as it can cause a force merge performing two merges.
+        engine = createEngine(copy(engine.config(), newMergePolicy(random(), false)));
         assertThat("Empty engine does not need flushing", engine.shouldPeriodicallyFlush(), equalTo(false));
         ParsedDocument doc =
             testParsedDocument(Integer.toString(0), null, testDocumentWithTextField(), SOURCE, null);