tests: Use test-desktop-shell for devices-test

It doesn't need to be using desktop-shell; trying to use it is not
sensible as it will try to bind to all the devices we're repeatedly
creating and destroying, sometimes lose the race, and fail the test
because desktop-shell has died too early.

Signed-off-by: Daniel Stone <daniels@collabora.com>
(cherry picked from commit ed97387a)