[autotests] Try to make TestWindowManagement more robust

The test started to segfault on build.kde.org. It looks like an event
survives the destruction of the server. Thus this change tries to
dispatchEvents once more before destroying everything.

src/wayland/autotests/client/test_wayland_windowmanagement.cpp

@@ -226,6 +226,7 @@ void TestWindowManagement::cleanup()
     }
     if (m_thread) {
         if (m_connection) {
+            m_connection->flush();
             m_connection->deleteLater();
         }
         m_thread->quit();
@@ -235,6 +236,7 @@ void TestWindowManagement::cleanup()
     }
     m_connection = nullptr;
 
+    m_display->dispatchEvents();
     delete m_windowManagementInterface;
     m_windowManagementInterface = nullptr;