drm/ttm/tests: depend on UML || COMPILE_TEST

At least the device test requires that no other driver using TTM is
loaded. So make those unit tests depend on UML || COMPILE_TEST to
prevent people from trying them on bare metal.

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://lore.kernel.org/all/20240219230116.77b8ad68@yea/