test_ring_buffer: Use yield instead of sleep_for(10us).

[kinetiknz]

See issue #567 for details.

@padenot, this retains (I think) the original intention, but the tests pass without yield too, so I'd be just as happy to remove it entirely.

[padenot]

Thanks. I'll try intentionally breaking the queue (for example putting a relaxed instead of an acquire or something), this should work (as in, correctly detect that it's now incorrect), this was one of the bugs I had when writing it, and was discovered with this sort of stress test.

I've been investigating other ways to test this kind of scenarios, because we'll need to remove a bunch of locks in gecko (and maybe cubeb, although it's not quite as problematic) this year, I'll follow up, this test kinda works but it's crude.