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>