display-test: test if threads are woken up on EAGAIN

When wl_connection_read() in wl_display_read_events() returns with EAGAIN,
we want the sleeping threads to be woken up. Test it!

Signed-off-by: Marek Chalupa <mchqwerty@gmail.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>