tests: Replace Q_OS_MACX -> Q_OS_OSX

Use the correct identifier for the OS X operating system.

Change-Id: I7158a6b77e5e7418bc6b0a565f003500820a346d
Reviewed-by: Jake Petroules <jake.petroules@petroules.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>