gh-149879: Fix multiprocessing resource tracker tests on Cygwin (#150440)

Skip test_resource_tracker_sigterm() if signal.pthread_sigmask() is
missing.

Author: vstinner

Lib/test/_test_multiprocessing.py

@@ -6310,6 +6310,8 @@ def test_resource_tracker_sigint(self):
         # Catchable signal (ignored by semaphore tracker)
         self.check_resource_tracker_death(signal.SIGINT, False)
 
+    @unittest.skipUnless(hasattr(signal, 'pthread_sigmask'),
+                         'need signal.pthread_sigmask')
     def test_resource_tracker_sigterm(self):
         # Catchable signal (ignored by semaphore tracker)
         self.check_resource_tracker_death(signal.SIGTERM, False)