bpo-31479: Always reset the signal alarm in tests (GH-3588) (GH-7314)

bpo-31479: Always reset the signal alarm in tests (GH-3588) (GH-7314)

* bpo-31479: Always reset the signal alarm in tests

Use "try: ... finally: signal.signal(0)" pattern to make sure that
tests don't "leak" a pending fatal signal alarm.

* Move two more alarm() calls into the try block

Fix also typo: replace signal.signal(0) with signal.alarm(0)

* Move another signal.alarm() into the try block

(cherry picked from commit 9abee722d448c1c00c7d4e11ce242ec7b13e5c49)