bpo-42641: Enhance test_select.test_select() (GH-23782)

Enhance test_select.test_select(): it now takes 500 ms rather than 10
seconds.

* Use Python rather than a shell as the child process to make the
  test more portable.
* Use a sleep of 50 ms per line rather than 1 second.
* Use subprocess.Popen rather than os.popen().