Issue #12363: improve siginterrupt() tests

Issue #12363: improve siginterrupt() tests

Backport commits 968b9ff9a059 and aff0a7b0cb12 from the default branch to 3.2
branch. Extract of the changelog messages:

"The previous tests used time.sleep() to synchronize two processes. If the host
was too slow, the test could fail.

The new tests only use one process, but they use a subprocess to:

- have only one thread
- have a timeout on the blocking read (select cannot be used in the test,
select always fail with EINTR, the kernel doesn't restart it)
- not touch signal handling of the parent process"

and

"Add a basic synchronization code between the child and the parent processes:
the child writes "ready" to stdout."

I replaced .communicate(timeout=3.0) by an explicit waiting loop using
Popen.poll().