AutoTest: Cancel possible running tasks on shutdown

If tasks are running while shutting down we might end up in a crash,
so cancel all tasks and handle possible invalid accesses of the
current running processing.

Change-Id: I69f7cac5f44390e322fa301af6d6794270c95c2a
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/autotest/autotestplugin.cpp

@@ -154,6 +154,7 @@ void AutotestPlugin::extensionsInitialized()
 
 ExtensionSystem::IPlugin::ShutdownFlag AutotestPlugin::aboutToShutdown()
 {
+    TestTreeModel::instance()->parser()->aboutToShutdown();
     return SynchronousShutdown;
 }