`stress_recv_timeout_two_threads`, in the mpmc and mpsc testsuites,
is a stress test of the `recv_timeout` function. This test processes and
ignores timeouts, and just ensures that every sent value gets received.
As such, the exact length of the timeouts is not critical, only that
the timeout and sleep durations ensure that at least one timeout
occurred.
The current tests have 100 iterations, half of which sleep for 200ms,
causing the test to take 10s. This represents around 2/3rds of the
*total* runtime of the `library/std` testsuite.
Reduce this to 50 iterations where half of them sleep for 10ms, causing
the test to take 0.25s.
Add a check that at least one timeout occurred.
This removes two minor OnceLock tests which test private methods. The
rest of the tests should be more than enough to catch mistakes in those
private methods. Also makes ReentrantLock::try_lock public. And finally
it makes the mpmc tests actually run.
