Avoid chromium assert in desktop capture tests

The page need to be visible to use desktop capture.

Task-number: QTBUG-71005
Change-Id: Icf19b68fb9132e528d206ac45245121beb31a1ac
Reviewed-by: Jüri Valdmann <juri.valdmann@qt.io>
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>