QCapturableWindows, Windows: Stop exposing windows that fail capture

The Windows backend for QCapturableWindow would not filter out windows
that cannot be captured. All QCapturableWindows returned by
QWindowCapture::capturableWindows() need to be valid and be working for
capture.

This patch adds stricter filtering when enumerating candidate windows
for capture.

This allows us to unblacklist a test.

Pick-to: 6.12
Change-Id: I784c984c351352acb6ec2310e8422dfeafc5aa25
Reviewed-by: Mikko Hallamaa <mikko.hallamaa@qt.io>
Reviewed-by: Tim Blechmann <tim.blechmann@qt.io>