test: fixed crash in tst_qmenubar::accel in qpa

test: fixed crash in tst_qmenubar::accel in qpa

Always verify that the application has an active window before
attempting to generate a key click.

This problem potentially affected all platforms, but is rare on non-qpa
Linux since QTest::qWaitForWindowShown would usually avoid the problem
on that platform.

Change-Id: I0c30be6228361faffa121c9c2d7a667f8351ebd6
Reviewed-on: http://codereview.qt.nokia.com/2896
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Kalle Lehtonen <kalle.ju.lehtonen@nokia.com>