gh-149879: Fix test_grp on Cygwin (#150495)

Author: vstinner

Lib/test/test_grp.py

@@ -2,6 +2,7 @@
 
 import random
 import string
+import sys
 import unittest
 from test.support import import_helper
 
@@ -35,7 +36,15 @@ def test_values_extended(self):
             self.skipTest('huge group file, extended test skipped')
 
         for e in entries:
-            e2 = grp.getgrgid(e.gr_gid)
+            try:
+                e2 = grp.getgrgid(e.gr_gid)
+            except KeyError:
+                # On Cygwin, some groups returned by getgrall() cannot be
+                # retrieved by getgrgid()
+                if sys.platform == 'cygwin':
+                    continue
+                raise
+
             self.check_value(e2)
             self.assertEqual(e2.gr_gid, e.gr_gid)
             name = e.gr_name