make timeout robust against budget-depleting tasks

[BraulioVM]

Refs: #4291 , #4300

Motivation

Up until now, if the future that we were applying a timeout to
consistently depleted the coop budget, the timeout never got a chance to
be evaluated. In the next call to poll, the underlying future would be
polled and it would once again deplete the budget. In those circumstances,
timeouts would not be respected. This can be surprising to people, and
in fact it was in #4291

Solution

The solution is to make a budget exception with timeout if it was the
underlying future that depleted the budget.

I guess there are still pathological cases where the budget is almost depleted
before polling the delay, and it gets depleted while polling the delay, resulting in
the timeout never being hit as well. How do we want to go about that? I could simply do:

if had_budget_before {
  coop::with_unconstrained(poll_delay)  
}

instead of taking into consideration whether the underlying future depleted (or not) the budget.

Testing

I have added a test along the lines of the original #4291 issue.

[BraulioVM]

The solution was inspired by @Darksonn 's comment in #4300 (comment)

[BraulioVM]

Anything else I can do here?

[Darksonn]

No, thanks.