Fix tst-wait-for.cc to use the new <osv/clock.hh> APIs.

This test uses std::abs on a time duration, and unfortunately C++11 fails
to implement std::abs on an std::chrono::duration. This patch also adds
support for this (in the form of a trivial template function) to
<osv/clock.hh>.

Reviewed-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>