Refactor epoll tests to use errno_result and improve notification checks

[hulxv]

Related to #4725

[rustbot]

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[RalfJung]

As discussed in the other PRs, marking as "waiting for author" until the first PRs landed, to avoid repeating the same review comments. We will get them all landed if you stick around, we should just take things one after the other. :)
@rustbot author

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[hulxv]

@rustbot ready

[RalfJung]

Thanks! This is a good first step for these tests. However, they are not done yet: the check_epoll_wait function in "no-blocking.rs" should be removed and replaced by the one from utils; there are many calls to write_all that should use one of the fancier helpers; there are a bunch more opportunities to use epoll_ctl_add. This can be done in a follow-up PR. :)