Tests: add trivial sleep test

Add a trivial sleep() test, which sleep()s for 2 seconds, and verifies
that this finishes and has slept for roughly 2 seconds.

I used this for debugging issue #26 - the attempts there ruined timers,
and in particular this trivial test hangs, as sleep() never returns.

(A note to our future automatic testing implementor: We need to allow
for the possibility that a test doesn't cleanly fail, but rather hangs,
and consider this a failure too).

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>