[3.11] gh-103186: Fix or catch 'extra' stderr output from unittests (#103196) (#106606)

Reduce test noise by fixing or catching and testing stderr messages from individual tests.

test_cmd_line_script.test_script_as_dev_fd calls spawn_python and hence subprocess.Popen with incompatible arguments. On POSIX, pass_fds forces close_fds to be True (subprocess.py line 848). Correct the call.

test_uuid.test_cli_namespace_required_for_uuid3: when the namespace is omitted, uuid.main calls argparse.Argument_Parser.error, which prints to stderr before calling sys.exit, which raises SystemExit. Unittest assertRaises catches the exception but not the previous output. Catch the output and test it.

test_warnings.test_catchwarnings_with_simplefilter_error similarly prints before raising. Catch the output and test it.
---------

Co-authored-by: Oleg Iarygin <[email protected]>
(cherry picked from commit 9d58225)

Author: terryjreedy

Lib/test/test_cmd_line_script.py

@@ -777,7 +777,7 @@ def test_script_as_dev_fd(self):
         with os_helper.temp_dir() as work_dir:
             script_name = _make_test_script(work_dir, 'script.py', script)
             with open(script_name, "r") as fp:
-                p = spawn_python(f"/dev/fd/{fp.fileno()}", close_fds=False, pass_fds=(0,1,2,fp.fileno()))
+                p = spawn_python(f"/dev/fd/{fp.fileno()}", close_fds=True, pass_fds=(0,1,2,fp.fileno()))
                 out, err = p.communicate()
                 self.assertEqual(out, b"12345678912345678912345\n")
 

Lib/test/test_warnings/__init__.py

@@ -388,9 +388,13 @@ def test_catchwarnings_with_simplefilter_error(self):
             with self.module.catch_warnings(
                 module=self.module, action="error", category=FutureWarning
             ):
-                self.module.warn("Other types of warnings are not errors")
-                self.assertRaises(FutureWarning,
-                                  self.module.warn, FutureWarning("msg"))
+                with support.captured_stderr() as stderr:
+                    error_msg = "Other types of warnings are not errors"
+                    self.module.warn(error_msg)
+                    self.assertRaises(FutureWarning,
+                                      self.module.warn, FutureWarning("msg"))
+                    stderr = stderr.getvalue()
+                    self.assertIn(error_msg, stderr)
 
 class CFilterTests(FilterTests, unittest.TestCase):
     module = c_warnings