tests should protect concurrent access to signal handlers

[asomers]

tests/sys/test_aio.rs has two separate tests that install handlers for SIGUSR2. This was probably the root cause of #529 . It should've been fixed by 0e3ef36, but we still sometimes see related failures. Instead of globally restricting the thread count to 1, those tests should use a mutex to protect access to signal handlers. So far, there are no tests outside of that file that also use SIGUSR2.