Add test for URL.createObjectURL on custom schemes

Works with registered custom schemes, does not work with unregistered ones. In
the latter case the function call is treated as a security violation and the
renderer process is killed.

Task-number: QTBUG-70420
Change-Id: I9c6fc0f02b44854bbceaffd1efbfe065dee61582
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>