Consumers of loopingcall may wish to exercise their code in test cases
without incurring actual wall clock sleep time in their test runs.
Heretofore, this required digging into the internals of the loopingcall
module and mocking something private (or something hidden by something
private).
This patch exposes a public oslo_service.fixture.SleepFixture for this
purpose. It can be maintained opaquely as internals change without
affecting its consumers.
See [1] for (one example of) the motivation behind this change.
[1] https://review.openstack.org/#/c/615724/
Change-Id: I0089c7778957456db66599abffaaad3a5332243c
b85d9353fb