test_loopback fails on Windows

[achronop]

1. test_loopback is failing with an assert:

Assertion failed: out_len == output_frame_count, file z:/build/build/src/media/libcubeb/src/cubeb_resampler_internal.h, line 257

2. test_ring_buffer hangs

[kinetiknz]

test_ring_buffer appears hung, but it's just making progress extremely slowly. test_ring_multi is using std::this_thread::sleep_for to sleep for 10us, which I assume is intended to give the other test thread time to run. With Windows' default timer frequency, each sleep is probably taking ~15.6ms. Even calling timeBeginPeriod/timeEndPeriod doesn't help much; doing ~720000 ~1ms sleeps still takes a long time. On Linux we sleep ~34us and on macOS we get 10us.

Replacing sleep_for with yield, the test passes on my Windows 10 machine in ~2.5s. That change also drops the test duration from ~25s to ~1s on my Linux machine and ~7s to ~0.6s on macOS.