gh-156376: Fix test_environ_path_cwd when NoDefaultCurrentDirectoryInExePath is set - #161277

test_shutil.test_environ_path_cwd on Windows relied on the user's
NoDefaultCurrentDirectoryInExePath environment setting and was therefore
non-deterministic. When that variable is set, Windows skips searching the
current directory, so shutil.which() returned 'file.exe' instead of the
expected '.\\file.exe' and the test failed.

This change mocks shutil._win_path_needs_curdir to return True for the
part of the test that expects the current-directory lookup, so the result no
longer depends on the machine's settings. test_cwd_win32 already uses the
same approach. On non-Windows platforms which() never calls that function,
so behavior there is unchanged.

Tested on Windows 11 (debug build):

• With NoDefaultCurrentDirectoryInExePath=1: before the fix,
  test_environ_path_cwd failed in both TestWhich and TestWhichBytes.
  After the fix, all 54 which tests pass (2 skipped as non-win32).
• Without the variable: all 54 which tests pass.
• Full test_shutil: 238 run, 0 failures.

No other which tests were affected by the variable.

• Issue: test_shutil.test_environ_path_cwd fails when NoDefaultCurrentDirectoryInExePath is set #156376