[3.5] bpo-30320, bpo-25277: backport test_eintr enhancements from master to 3.5 ...

[3.5] bpo-30320, bpo-25277: backport test_eintr enhancements from master to 3.5 (#1532)

* bpo-30320: test_eintr now uses pthread_sigmask() (#1523)

Rewrite sigwaitinfo() and sigtimedwait() unit tests for EINTR using
pthread_sigmask() to fix a race condition between the child and the
parent process.

Remove the pipe which was used as a weak workaround against the race
condition.

sigtimedwait() is now tested with a child process sending a signal
instead of testing the timeout feature which is more unstable
(especially regarding to clock resolution depending on the platform).
(cherry picked from commit 211a392cc15f9a7b1b8ce65d8f6c9f8237d1b77f)

* test_eintr: Fix ResourceWarning warnings

(cherry picked from commit c50cccfcc3b3a9ef3fe7a78b7e7271930dc24aee)

* test_eintr: remove unused import

* bpo-25277: Add a watchdog to test_eintr

Set a timeout of 10 minutes in test_eintr using faulthandler.