Some test_eintr hangs on waiting for the child process completion if an error
occurred on the parent. Kill the child process on error (in the parent)
to avoid the hang.

Reported by Jakub Wilk.

Reported by Jakub Wilk.

Reported by Jakub Wilk.

* Add verbose mode to test_eintr
* Always enable verbose mode in test_eintr
* Use faulthandler.dump_traceback_later() with a timeout of 15 minutes in
  eintr_tester.py

Add an unit test on os.waitpid()

Fix the regex to support the version 7.10: minor version with two digits

Patch by me and Elvis Pranskevichus

(we'll make a couple more commits tomorrow before release)