gh-156376: Make test_environ_path_cwd independent of the ambient environment

shutil.which() only inserts the current directory into the search path when
_winapi.NeedCurrentDirectoryForExePath() returns true, and that consults the
NoDefaultCurrentDirectoryInExePath environment variable. On a machine where
that variable is set, TestWhich.test_environ_path_cwd and
TestWhichBytes.test_environ_path_cwd failed because which() correctly
declined to search the current directory.

Unset the variable inside the EnvironmentVarGuard the test already uses, so
an ambient value cannot change what the test exercises.

Author: KindPinningFinalBoss

Lib/test/test_shutil.py

@@ -2626,6 +2626,12 @@ def test_environ_path_cwd(self):
         # PATH=':': explicitly looks in the current directory
         with os_helper.EnvironmentVarGuard() as env:
             env['PATH'] = os.pathsep
+            if sys.platform == "win32":
+                # shutil.which() only searches the current directory when
+                # _winapi.NeedCurrentDirectoryForExePath() returns true, which
+                # in turn consults NoDefaultCurrentDirectoryInExePath.  Unset
+                # it so an ambient value cannot change what is searched.
+                env.unset('NoDefaultCurrentDirectoryInExePath')
             with unittest.mock.patch('os.confstr', return_value=self.dir, \
                                      create=True), \
                  support.swap_attr(os, 'defpath', self.dir):