tests: Replace Q_OS_MACX -> Q_OS_OSX

Use the correct identifier for the OS X operating system.

Change-Id: Iff433d312c7c808ddce13466be3db628cf3a9890
Reviewed-by: Jake Petroules <jake.petroules@petroules.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>