refactor: drop the _sleep_once fixture

[tonyandrewmeyer]

_sleep_once returns a closure that sleeps on its first call and does nothing after that. Its docstring explains the point: "This allows fixtures of the same scope to ensure a single sleep happens before teardown." There is only one such fixture now, juju_factory, and it calls it once in its own teardown, so there is nothing for the guard to guard against. The fixture is module-scoped too, so slept is recreated per module and can never be True when that one call happens.

Replace it with a plain time.sleep(_LOG_WAIT) at the call site. Same total sleep count, one, in the same place.

Fixes #107