Unskip QWebEngineSettings::JavascriptCanOpenWindows related tests

Window.showModalDialog() is not supported since Chromium 37, so we can
remove the showModalDialog test case.

If the view is not visible, the width and the height of the requested
geometry is always 0 in tst_QWebEnginePage::openWindowDefaultSize.

Change-Id: I420828815bc9528070ca58a9e42dea44517aeafc
Reviewed-by: Andras Becsi <andras.becsi@digia.com>