tests: Support weston_test request for adding a test seat

tests: Support weston_test request for adding a test seat

Support adding a test seat using the weston_test.device_add request.
This will be used in tests in upcoming commits where we will need to
re-add the seat after having it removed.

We only support one test seat at the moment, so this commit also
introduces checks to ensure the client doesn't try to create multiple
test seats or try to remove an already removed test seat.

Signed-off-by: Alexandros Frantzis <alexandros.frantzis@collabora.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>