`test_subprocess` is flaky under `PYTHONWARNINGS=ignore`

Feature or enhancement

Proposal:

make test

my env has a export PYTHONWARNINGS=ignore

make test will fall

FAIL: test_encoding_warning (test.test_subprocess.RunFuncTestCase.test_encoding_warning)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/hyi/prs/cpython/Lib/test/test_subprocess.py", line 1837, in test_encoding_warning
    self.assertEqual(len(lines), 2, lines)
    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^
AssertionError: 0 != 2 : []

at first I thought it is not a big deal

but I checked the code

class Test_TextTestRunner(unittest.TestCase):
    """Tests for TextTestRunner."""

    def setUp(self):
        # clean the environment from pre-existing PYTHONWARNINGS to make
        # test_warnings results consistent
        self.pythonwarnings = os.environ.get('PYTHONWARNINGS')
        if self.pythonwarnings:
            del os.environ['PYTHONWARNINGS']

    def tearDown(self):
        # bring back pre-existing PYTHONWARNINGS if present
        if self.pythonwarnings:

existing code can aviod this.
so I thought it may be can enhance

Has this already been discussed elsewhere?

No response given

Links to previous discussion of this feature:

No response

Linked PRs

• gh-138637

Start with Lib/test/test_subprocess.py at RunFuncTestCase.test_encoding_warning, then review the shown PYTHONWARNINGS cleanup pattern in the relevant test code. Run make test with PYTHONWARNINGS=ignore and verify the subprocess warning test produces the expected two lines without disturbing the existing environment handling.