Issue #25764: Skip the test on OS X

The OS X buildbots were failing at the second setrlimit() call with EPERM, as
if they were trying to raise the hard limit. The call should be keeping the
hard limit the same and raising the soft limit back to its original value, so
I don't understand the failure.

Lib/test/test_subprocess.py

@@ -1416,6 +1416,8 @@ class POSIXProcessTestCase(BaseTestCase):
             if not enabled:
                 gc.disable()
 
+    @unittest.skipIf(
+        sys.platform == 'darwin', 'setrlimit() seems to fail on OS X')
     def test_preexec_fork_failure(self):
         # The internal code did not preserve the previous exception when
         # re-enabling garbage collection