bpo-33613, test_semaphore_tracker_sigint: fix race condition (#7850)

Fail `test_semaphore_tracker_sigint` if no warnings are expected and one is received.

Fix race condition when the child receives SIGINT before it can register signal handlers for it.

The race condition occurs when the parent calls
`_semaphore_tracker.ensure_running()` (which in turn spawns the
semaphore_tracker using `_posixsubprocess.fork_exec`), the child
registers the signal handlers and the parent tries to kill the child.
What seem to happen is that in some slow systems, the parent sends the
signal to kill the child before the child protects against the signal.