Make tst_qwindow::positioning more reliable

Don't check for absolute counts of resize events but just check if we
were resized. Also use QTest::qWaitForWindowExposed instead of QTRY_COMPARE
and checking for QEvent::Expose.

Change-Id: Ie383493a8ce6d88cad50bd6375d432ad1578449c
Reviewed-by: Jørgen Lind <jorgen.lind@theqtcompany.com>