Add initial test_env_vars_for_windows_tests

The new test method just verifies the current behavior of the
HIDE_WINDOWS_KNOWN_ERRORS and HIDE_WINDOWS_FREEZE_ERRORS
environment variables. This is so there is a test to modify when
changing that behavior. The purpose of these tests is *not* to
claim that the behavior of either variable is something code that
uses GitPython can (or has ever been able to) rely on.

This also adds pytest-subtests as a dependency, so multiple
failures from the subtests can be seen in the same test run.

test-requirements.txt

@@ -7,4 +7,5 @@ pre-commit
 pytest
 pytest-cov
 pytest-instafail
+pytest-subtests
 pytest-sugar

test/test_util.py

@@ -4,12 +4,14 @@
 # This module is part of GitPython and is released under
 # the BSD License: https://opensource.org/license/bsd-3-clause/
 
+import ast
 import contextlib
 from datetime import datetime
 import os
 import pathlib
 import pickle
 import stat
+import subprocess
 import sys
 import tempfile
 import time
@@ -502,3 +504,46 @@ def test_remove_password_from_command_line(self):
 
         assert cmd_4 == remove_password_if_present(cmd_4)
         assert cmd_5 == remove_password_if_present(cmd_5)
+
+    @ddt.data("HIDE_WINDOWS_KNOWN_ERRORS", "HIDE_WINDOWS_FREEZE_ERRORS")
+    def test_env_vars_for_windows_tests(self, name):
+        def run_parse(value):
+            command = [
+                sys.executable,
+                "-c",
+                f"from git.util import {name}; print(repr({name}))",
+            ]
+            output = subprocess.check_output(
+                command,
+                env=None if value is None else dict(os.environ, **{name: value}),
+                text=True,
+            )
+            return ast.literal_eval(output)
+
+        assert_true_iff_win = self.assertTrue if os.name == "nt" else self.assertFalse
+
+        truthy_cases = [
+            ("unset", None),
+            ("true-seeming", "1"),
+            ("true-seeming", "true"),
+            ("true-seeming", "True"),
+            ("true-seeming", "yes"),
+            ("true-seeming", "YES"),
+            ("false-seeming", "0"),
+            ("false-seeming", "false"),
+            ("false-seeming", "False"),
+            ("false-seeming", "no"),
+            ("false-seeming", "NO"),
+            ("whitespace", " "),
+        ]
+        falsy_cases = [
+            ("empty", ""),
+        ]
+
+        for msg, env_var_value in truthy_cases:
+            with self.subTest(msg, env_var_value=env_var_value):
+                assert_true_iff_win(run_parse(env_var_value))
+
+        for msg, env_var_value in falsy_cases:
+            with self.subTest(msg, env_var_value=env_var_value):
+                self.assertFalse(run_parse(env_var_value))