tests: Add Lolex-based unit test of `sleep`.
tests: Add Lolex-based unit test of `sleep`.

Test that `sleep` has ideal behavior in the presence of an ideal
clock.

The original test remains in place, to test that `sleep` also has
reasonably robust behavior in the presence of a non-ideal clock.

vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile