tests: Replace Q_OS_MACX -> Q_OS_OSX

Use the correct identifier for the OS X operating system.

Change-Id: I5f93d63fc7e3041a45485ca10ccf5a1bde04bb01
Reviewed-by: Jake Petroules <jake.petroules@petroules.com>
Reviewed-by: Eike Ziller <eike.ziller@digia.com>