clock.advance(0) will run not only any calls scheduled for clock.seconds() or
earlier, but it will also run any calls that _those_ calls schedule with
`clock.callLater(0, ...)`.

This diverges from the real IReactorTime implementations which force all
scheduled events to wait at least one reactor iteration before they can run.

This impacts the LoopingCall test cases for the case of interval == 0 because
those tests do a lot of scheduling with `clock.callLater(0, ...)`.  In
particular, the tests often `advance(0)` and expect a `LoopingCall` to execute
multiple iterations.

These changes make the tests advance the clock in enough discrete steps that a
real reactor would have time to run all of the necessary calls.

After twisted#5962 the old version of these tests will fail.  Also as part of twisted#5962
one clearly-marked fudge left in these tests will need to be removed because I
don't see how to make the test work (and remain precise) with both the broken
Clock behavior and the real reactor behavior.

Co-authored-by: Adi Roiban <adiroiban@gmail.com>