winmm/tests: Don't test the non-default Midi devices if the 'PreferredOnly' Windows setting is set.

Hi,

a few month ago when the discussion about "PreferredOnly" registry
setting came up, I queried Google and found that many more users
use this setting than one may believe - i.e. it is not exotic.  Therefore,
I don't think it's that great an idea to reduce the number of devices that
the tests stress using that method: we'll get less feedback from a number
of real machines.
OTOH, I have no solution to the vmware MIDI trouble on Francois' machines.

Regards,
 Jörg Höhle