channel: ready::fairness2 test is racy

[jonhoo]

The test occasionally fails in CI and locally.

Reproduce by running:

$ for (( i=1 ;; i+=1 )); do echo "*** test $i ***"; cargo test --test ready fairness || break; done

To quote @cuviper:

It seems to fail even more frequently with --release, so maybe the test just needs more allowance?

Hmm yeah that definitely looks like a fluke - statistically speaking, the test is bound to fail :) (but we hope so rarely it never actually fails). I think bumping the threshold should do the trick.

[jonhoo]

I tried changing the assertion to

assert!(hits.iter().all(|x| x.get() > 0));

And unfortunately even that sometimes fails. I guess we'd have to increase the count?

[jonhoo]

I opened #495 with the updated assertion and an increased COUNT, and that seems to not be racy for me at least.