tests: add test for reading after an error occurred

This test shows that it's possible to successfully call wl_display_prepare_read
and wl_display_read_events after an error occurred. That may lead to
deadlock.

When you call prepare read from two threads and then call read_events,
one thread gets sleeping. The call from the other thread will return -1 and invokes
display_fatal_error, but since
we have display->last_error already set, the broadcast is not called and
the sleeping thread sleeps indefinitely.

Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>