Add test for epoll

[tiif]

I forgot how the implementation works and was wondering how the assumption here is being upheld. So I wrote a test.

miri/src/shims/unix/linux/epoll.rs

Lines 541 to 542 in 9d9da34

	// Edge-triggered notification only notify one thread even if there are
	// multiple threads block on the same epfd.

This is mainly written for me to verify if it actually works, so it is perfectly ok to not merge this.

[tiif]

@rustbot ready

[RalfJung]

Don't our existing tests already cover this case? I added a comment along those lines:

miri/tests/pass-dep/libc/libc-epoll-no-blocking.rs

Lines 83 to 84 in 9e0205d

	// Check that this is indeed using "ET" (edge-trigger) semantics: a second epoll should return nothing.
	check_epoll_wait::<8>(epfd, &[]);

The test is not very clear about the fact that that is what it tests, though.

[RalfJung]

Ah I guess this is a different aspect of "edge semantics".

[tiif]

Ah I guess this is a different aspect of "edge semantics".

Yup it's different. The non-blocking test checks "if no event occurs between two epoll_wait calls, we should not receive any notification during the second epoll_wait". The test in this PR checks "if multiple threads are blocking on the same epoll fd, a single notification on that epoll fd should wake up only one thread (the alternative would be waking up all threads, i.e. thundering herd behaviour )"