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