winrt: Use config feature when checking for winrt backend in auto tests

As the winrt backend may also be used on desktop Windows, checking for
the operating system does no longer work.

While touching these lines deprecated OS X checks were fixed as well.

Task-number: QTBUG-61566
Change-Id: I247979aa63de6a71ac4908bda946d7db1e29b6c3
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>
Reviewed-by: Alex Blasche <alexander.blasche@qt.io>