tst_QMdiArea::subWindowActivated2(): Add activateWindow().

The widget is hidden, reshown and tested for active window, which
is flaky. Add a call to activateWindow() which increases
the chances of it becoming the active window should another window
appear.

Change-Id: Ibbecdbc43e2ac9638aec497c47fffaaffa1855af
Reviewed-by: Simon Hausmann <simon.hausmann@theqtcompany.com>

tests/auto/widgets/widgets/qmdiarea/tst_qmdiarea.cpp

@@ -497,6 +497,7 @@ void tst_QMdiArea::subWindowActivated2()
     spy.clear();
 
     mdiArea.show();
+    mdiArea.activateWindow();
     QVERIFY(QTest::qWaitForWindowActive(&mdiArea));
     QTRY_COMPARE(spy.count(), 1);
     QVERIFY(mdiArea.currentSubWindow());