Instead of racily advancing time forward, this commit attempts to rework
how WaitFor works, such that it makes advances when all sleeper futures
that have been created have been polled (by handing the MockSleepRuntime
a Waker with which to wake up the WaitFor).
The above described mechanics work well enough for the double timeout
test, but fail in the presence of code that spawns asynchronous /
background tasks that must make progress before time is advanced for the
test to work properly. In order to deal with these cases, a set of APIs
are introduced in order to block time from being advanced until some
code has run, and a carveout added in order to permit small advances in
time where required.
(In some cases, code needed to be hacked up a bit in order to be made
properly testable using these APIs; the `MockablePlan` trait included in
here is somewhat unfortunate.)
This should fix arti#149.
eta
Nick Mathewson
Daniel Eades