TEST: Ensure a manual flush always occur (#36188)

The test testLookupSeqNoByIdInLucene fails because it expects if any
change should be visible after a flush. However, that flush might be
ignored if the waitIfOngoing parameter is false (the default value), and
there is an ongoing flush triggered after merge is running.

Closes #35823

Author: dnhatn

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

@@ -3780,7 +3780,7 @@ public void testLookupSeqNoByIdInLucene() throws Exception {
                     lookupAndCheck.run();
                 }
                 if (rarely()) {
-                    engine.flush();
+                    engine.flush(false, true);
                     lookupAndCheck.run();
                 }
             }