Eliminate some qWait() calls from QWebEnginePage auto tests

The eliminated qWait() calls were used for waiting for a TestPage to
create a new window. To avoid waiting for an arbitrary time these
calls are replaced by waiting for Testpage::windowCreated() signal.

Change-Id: I96a142f9b99c4a36f1bf36ecba06c7ef6be732ba
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>