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)
diff --git a/Lib/test/test_threadsignals.py b/Lib/test/test_threadsignals.py
index 2f7eb60..3d79fd5 100644
--- a/Lib/test/test_threadsignals.py
+++ b/Lib/test/test_threadsignals.py
@@ -52,9 +52,11 @@
         # wait for it return.
         if signal_blackboard[signal.SIGUSR1]['tripped'] == 0 \
            or signal_blackboard[signal.SIGUSR2]['tripped'] == 0:
-            signal.alarm(1)
-            signal.pause()
-            signal.alarm(0)
+            try:
+                signal.alarm(1)
+                signal.pause()
+            finally:
+                signal.alarm(0)
 
         self.assertEqual( signal_blackboard[signal.SIGUSR1]['tripped'], 1)
         self.assertEqual( signal_blackboard[signal.SIGUSR1]['tripped_by'],