test_shutil.test_environ_path_cwd fails when NoDefaultCurrentDirectoryInExePath is set

Bug description:

test.test_shutil.TestWhich.test_environ_path_cwd and
test.test_shutil.TestWhichBytes.test_environ_path_cwd fail on Windows when the
environment variable NoDefaultCurrentDirectoryInExePath is set in the
environment the test suite runs in:

FAIL: test_environ_path_cwd (test.test_shutil.TestWhich.test_environ_path_cwd)
----------------------------------------------------------------------
AssertionError: '@test_21832_tmpæ.Exe' != '.\@test_21832_tmpæ.Exe'
- @test_21832_tmpæ.Exe
+ .\@test_21832_tmpæ.Exe

The test asserts that shutil.which() finds the file in the current directory,
expecting a .\-prefixed result on Windows. However, shutil.which() only
inserts the current directory into the search path when
_win_path_needs_curdir() returns true (Lib/shutil.py), and that delegates to
_winapi.NeedCurrentDirectoryForExePath(). The underlying
NeedCurrentDirectoryForExePathW() API consults
NoDefaultCurrentDirectoryInExePath, so when that variable is present the
current directory is not searched and the assertion fails.

The test does use os_helper.EnvironmentVarGuard, but only to set PATH; it
does not neutralise NoDefaultCurrentDirectoryInExePath, so an ambient value
leaks into the test. TestWhich.test_win_path_needs_curdir in the same file
already accounts for this mechanism by patching
_winapi.NeedCurrentDirectoryForExePath.

Reproducer on Windows:

set NoDefaultCurrentDirectoryInExePath=1
python -m test test_shutil

Unsetting the variable inside the existing EnvironmentVarGuard makes the test
independent of the ambient environment. I have a fix and will open a PR.

CPython versions tested on:

3.16 (main, commit 04242c027fe)

Operating systems tested on:

Windows 11

Linked PRs

• gh-156377

Start in Lib/test/test_shutil.py with TestWhich.test_environ_path_cwd and TestWhichBytes.test_environ_path_cwd, then compare TestWhich.test_win_path_needs_curdir. Run the affected test module on Windows with NoDefaultCurrentDirectoryInExePath set; done means these tests no longer depend on that ambient variable and pass in both environments.