Correct flush parameters in engine test

Since #40213, we forbid a combination of flush parameters: force=true
and wait_if_ongoing=false.

Closes #41236

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

@@ -3104,7 +3104,7 @@ public class InternalEngineTests extends EngineTestCase {
                                 break;
                             }
                             case "flush": {
-                                engine.flush(true, false);
+                                engine.flush(true, true);
                                 break;
                             }
                         }