TEST: Ensure a manual flush always occur

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 elastic#35823

Author: dnhatn

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

@@ -3809,7 +3809,6 @@ public void testSequenceIDs() throws Exception {
         searchResult.close();
     }
 
-    @AwaitsFix(bugUrl = "https://github.com/elastic/elasticsearch/issues/35823")
     public void testLookupSeqNoByIdInLucene() throws Exception {
         int numOps = between(10, 100);
         long seqNo = 0;
@@ -3878,7 +3877,7 @@ public void testLookupSeqNoByIdInLucene() throws Exception {
                     lookupAndCheck.run();
                 }
                 if (rarely()) {
-                    engine.flush();
+                    engine.flush(false, true);
                     lookupAndCheck.run();
                 }
             }