Tests: Emit coreAboutToClose() before exiting

Now address sanitizer will not show false positives.

Change-Id: Ieeb4dc31697920ed1ca542f5647f4cb0b23a29ab
Task-number: QTCREATORBUG-14713
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>

src/libs/extensionsystem/pluginmanager.h

@@ -158,6 +158,7 @@ signals:
 
     void pluginsChanged();
     void initializationDone();
+    void testsFinished(int failedTests);
 
 public slots:
     void remoteArguments(const QString &serializedArguments, QObject *socket);