tests: fix event_loop_timer_updates

It may happen that there's some time between the first and the other timer expire.
If epoll_wait is called after the first timer expired and
the other not, it returns only one source to dispatch and therefore
the test fails. To fix that, sleep a while before
wl_event_loop_dispatch() to make sure both timers expired.

To be 100% sure, we could use poll() before calling
wl_event_loop_dispatch(), but that would need modification in libwayland
(need to get the source's fd somehow)

https://bugs.freedesktop.org/show_bug.cgi?id=80594



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