test_posix: fix flaky chown-to-gid-0 assertion under gid 0

Summary:
`PosixTester._test_all_chown_common` asserts that chowning a file's group to
gid 0 raises OSError when the process is "unprivileged". It gated that on
`0 not in os.getgroups()`, but group membership for chown also includes the
process's effective gid, which getgroups() does not report. In sandboxes /
user namespaces that run with gid 0 and a non-zero uid, chgrp-to-0 therefore
succeeds and the assertion fails ("OSError not raised by {chown,fchown,lchown}")
while the owner-to-0 assertions (which need CAP_CHOWN) still correctly raise.

Also require `os.getegid() != 0` before asserting the failure.

<!-- pyimport-metadata:begin -->
This diff adds a patch to the `third-party/python/3.14` Meta-internal fork.

test_posix: chgrp-to-0 membership also includes the effective gid; avoids spurious failure when the sandbox runs with gid 0

Reviewed By: Yhg1s

Differential Revision: D108567108

fbshipit-source-id: 25345fc871b9e834af3241a94afed9cf85dcb987

Author: itamaro

Lib/test/test_posix.py

@@ -838,7 +838,13 @@ def check_stat(uid, gid):
             self.assertRaises(OSError, chown_func, first_param, 0, -1)
             check_stat(uid, gid)
             if hasattr(os, 'getgroups'):
-                if 0 not in os.getgroups():
+                # The kernel checks group membership for chown against the
+                # effective (fs) gid as well as the supplementary groups, so
+                # guard on getegid() too. In some sandboxes/user namespaces the
+                # process runs with an effective gid of 0 (and a non-zero uid),
+                # which makes chowning the group to 0 succeed even though 0 is
+                # absent from getgroups().
+                if 0 not in os.getgroups() and os.getegid() != 0:
                     self.assertRaises(OSError, chown_func, first_param, -1, 0)
                     check_stat(uid, gid)
         # test illegal types