tests: Run devices tests using the test desktop shell

tests: Run devices tests using the test desktop shell

Use the weston-test-desktop-shell to run the devices tests, instead of
the currently used desktop-shell. The test desktop shell doesn't
interact with temporary globals (e.g. wl_seat), thus avoiding an
inherent race in the current wayland protocol when removing globals.
This will allow us to safely add tests which add/remove such globals in
upcoming commits.

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