gh-149879: Fix multiprocessing resource tracker tests on Cygwin

[vstinner]

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

• Issue: Enhance Cygwin support #149879

[vstinner]

The test fails also on Linux if signal.pthread_sigmask() is missing. Apply the patch:

diff --git a/Lib/multiprocessing/resource_tracker.py b/Lib/multiprocessing/resource_tracker.py
index d3328a8c617..a2cfcb550a8 100644
--- a/Lib/multiprocessing/resource_tracker.py
+++ b/Lib/multiprocessing/resource_tracker.py
@@ -31,7 +31,7 @@
 
 __all__ = ['ensure_running', 'register', 'unregister']
 
-_HAVE_SIGMASK = hasattr(signal, 'pthread_sigmask')
+_HAVE_SIGMASK = False
 _IGNORED_SIGNALS = (signal.SIGINT, signal.SIGTERM)
 
 def cleanup_noop(name):

Run the test:

$ ./python -m test test_multiprocessing_spawn.test_misc -m test_resource_tracker_sigterm -v
...
Traceback (most recent call last):
  File "/home/vstinner/python/main/Lib/multiprocessing/resource_tracker.py", line 453, in main
    cache[rtype].remove(name)
    ~~~~~~~~~~~~~~~~~~~^^^^^^
KeyError: '/mp-1zhvb0mv'

======================================================================
FAIL: test_resource_tracker_sigterm (test.test_multiprocessing_spawn.test_misc.TestResourceTracker.test_resource_tracker_sigterm)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/vstinner/python/main/Lib/test/_test_multiprocessing.py", line 6315, in test_resource_tracker_sigterm
    self.check_resource_tracker_death(signal.SIGTERM, False)
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/vstinner/python/main/Lib/test/_test_multiprocessing.py", line 6307, in check_resource_tracker_death
    self.assertEqual(len(all_warn), 0)
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^
AssertionError: 1 != 0
...