tests: Synchronize client termination in idle callback

We currently wait for clients in the wl_client destroy signal, which is
called before the client is destructed and the socket is closed. If test
clients rely on being closed due to the socket being closed we'd dead
lock. Avoid this by synchronizing in an idle task that is called after
the client is fully destroyed.

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>