Fix path issues tst_QQuickWebEngineView

Compare URLs, not paths. This saves us a bunch of conversions and
fixes the test on Windows.

Change-Id: Iaf91e7be2b015545bb7b2b1141bc316f85e629ea
Reviewed-by: Michael Brüning <michael.bruning@theqtcompany.com>
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@theqtcompany.com>