sim: add unsupported caps to device creation

Update `make_device` to return a slice of unsupported caps for a test.
This allows skipping tests in devices that are known to be non working
under some build configuration.

The device constructor was updated to return a `Result`, so that the
specific reason for skipping can be returned as a `String`.

Signed-off-by: Fabio Utzig <utzig@apache.org>