tests: skip drmdevice test if the machine doesn't have any drm device

tests: skip drmdevice test if the machine doesn't have any drm device

Error message was invalid too, negative values aren't the number of
devices, they're errno error codes.

Signed-off-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>