tests: Handle removal of seat global in test clients

tests: Handle removal of seat global in test clients

The current test client code completely ignores removal of globals.
This commit updates the code to properly handle removal of globals in
general, and of seat globals in particular. This ensures that the test
client objects are in sync with the server and any relevant resources
are released accordingly.

This update will be used by upcoming tests to check that seat removal
and re-addition is working properly.

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