bpo-35347: Cleanup test_socket.NonBlockingTCPTests (GH-10818)

* Replace testInheritFlags() with two tests:
  testInheritFlagsBlocking() and testInheritFlagsTimeout()
  to test different default socket timeout. Moreover, the test now
  checks sock.gettimeout() rather than a functional test on recv().
* Replace time.time() with time.monotonic()
* Add socket_setdefaulttimeout() context manager to restore the
  default timeout when the test completes.
* Remove testConnect(): accept() wasn't blocking and testAccept()
  already tests non-blocking accept().
* Remove accept() functional test from testInitNonBlocking():
  already tested by testAccept()
* Rewrite testSetBlocking() with a new assert_sock_timeout() method
* Use addCleanup() and context manager to close sockets
* Replace assertTrue(x < y) with assertLess(x, y)