bpo-30320: test_eintr now uses pthread_sigmask() (#1523) (#1524)

bpo-30320: test_eintr now uses pthread_sigmask() (#1523) (#1524)

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)