Close engine before reset log appender (#50390)

Merge threads can run and access the mock appender after we have stopped it.

Closes #50315

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

@@ -2376,7 +2376,7 @@ public class InternalEngineTests extends EngineTestCase {
             engine.index(indexForDoc(doc));
             engine.flush();
             assertTrue(mockAppender.sawIndexWriterMessage);
-
+            engine.close();
         } finally {
             Loggers.removeAppender(rootLogger, mockAppender);
             mockAppender.stop();