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>
winrt: Fix window resize

The fixed-size EGL display must be updated on PC as well as Windows Phone.

Task-number: QTBUG-43112
Change-Id: I0ba90370b67e8c065843e5754579e341b647266d
Reviewed-by: Oliver Wolff <oliver.wolff@theqtcompany.com>