[test] Addres TestConcurrentMany*.py flakiness on macOS

On "fast" macOS machines, the TestConcurrentMany*.py tests would fail
randomly with different numbers of breakpoints, watchpoints, etc. This
seems to be avoidable by giving the threads a little time to breath
after the passing the synchronization barrier. This is far from a
structural fix but it reduces the flakiness.

llvm-svn: 370785