bpo-35347: Fix test_socket.NonBlockingTCPTests (GH-10791)

testAccept() and testRecv() of test_socket.NonBlockingTCPTests have a
race condition: time.sleep() is used as a weak synchronization
primitive and the tests fail randomly on slow buildbots.

Use a reliable threading.Event to fix these tests.

Other changes:

* Replace send() with sendall()
* Expect specific BlockingIOError rather than generic OSError
* Add a timeout to select() in testAccept() and testRecv()
* Use addCleanup() to close sockets
* Use assertRaises()