ddraw/tests: Skip cubemap tests when cubemap isn't supported.

[Alexander Dorofeyev]

Stefan Dösinger wrote:
> DeviceLoadTest is quite big, does splitting up make sense? Otherwise, what
> you say is fair enough - I do not care too much about code beauty in the
> tests as long as the tests work(since one hardly ever reads the test or does
> lots of work on it, and the Windows apps we run are often crappy as well)

I don't care too much either way, but I don't see use in splitting because 
cubemap part uses same variables, works in a similar way to other non-cubemap 
parts and generally is part of the whole.