tests: Test that one can fetch the protocol error after EPIPE

If a client is terminated due to some reason, it should always be
possible to retrieve protocol error associated with the termination.
Test that, while either using the dispatch helpers
(wl_display_dispatch(_queue)() or the prepare read API, it should be
possible to retrieve the error after EPIPE.

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