event-loop-test: Confirm distant timers do not fire

This change expands the `event_loop_timer` test to use two different
timers with different timeouts; it now implicitly checks that e.g.
both timers do not expire at the same time, and that the first timer
expiring does not prevent the second from doing so. (While such failure
modes are unlikely with timer event sources based on individual
timerfds, they are possible when multiple timers share a common timerfd.)

Signed-off-by: Manuel Stoeckl <code@mstoeckl.com>