[3.14] gh-140482: Avoid changing terminal settings in test_pty (gh-142202) (gh-142239)

The previous test_spawn_doesnt_hang test had a few problems:

* It would cause ENV CHANGED failures if other tests were running
  concurrently due to stty changes
* Typing while the test was running could cause it to fail
(cherry picked from commit c0c65141b37029bfb364094a6dfb4c75ebf8359e)

Co-authored-by: Sam Gross <colesbury@gmail.com>