After taking a more detailed look at our commonly failing timeout
tests... turns out that most of them are useless as written and
don't test what they're supposed to.
This PR has a couple of changes:
* Tests for timeout in while/for/foreach should just have the loop
as an infinite loop. Calling into something like busy_wait means
that we just end up always testing whatever busy_wait does.
* Tests for timeouts in calls need to be based on something like
sleep, otherwise we'd have to introduce a loop, and we'd end up
testing timeout of the looping structure instead. Using sleep only
works on Windows, because that's the only system where sleep counts
towards the timeout. As such, many of those tests are now Windows only.
* Removed some tests where I don't see a good way to test what they're
supposed to test. E.g. how can we test a timeout in eval() specifically?
The shutdown function tests are marked as XFAIL, as we are currently
missing a timeout check in call_user_function. I believe that's a
legitimate issue.
Closes GH-4969.
