Skip OpenUrlJob test on Windows/macOS

OpenUrlJob somewhat works on these platforms because we delegate to
QDesktopServices::openUrl()

The test however is very centered around XDG things, (desktop files,
mimeapps.list etc), which doesn't exist in Windows/macOS.

The test therefore doesn't work on Windows/macOS, so skip it.

Given that most of the interesting code is in Qt anyway there's
not much to test anyway