Tests: Switch test apps to use usleep_safe()

usleep_safe() ensures that usleep honors its timing guarantees
(which it, according to the man pages, does not strictly enforce).

Some tests need to make ensure no events are emitted at the same
timestamp since Babeltrace does not define an ordering which can
be relied upon in such a case.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>