This allows these test modules to pass on builds/targets without
unwinding support, where `panic = "abort"` - the ignored tests are for
functionality that's not supported on those targets.
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.
