[3.6] bpo-31234: Enhance test_thread.test_forkinthread() (GH-3516) (#3519)

[3.6] bpo-31234: Enhance test_thread.test_forkinthread() (GH-3516) (#3519)

* test_thread.test_forkinthread() now waits until the thread completes.
* Check the status in the test method, not in the thread function
* Don't ignore RuntimeError anymore: since the commit
  346cbd351ee0dd3ab9cb9f0e4cb625556707877e (bpo-16500,
  os.register_at_fork(), os.fork() cannot fail anymore with
  RuntimeError.
* Replace 0.01 literal with a new POLL_SLEEP constant
* test_forkinthread(): test if os.fork() exists rather than testing
  the platform.
(cherry picked from commit a15d155aadfad232158f530278505cdc6f326f93)