Restore the tests command and deprecate access to the module.

[jaraco]

Summary of changes

As suggested by @hauntsaninja, add a deprecation warning to access of setuptools.command.test and raise an error when running the command.

This approach provides a gentler offboarding for projects still reliant on the presence of the module or the command. Currently, it gives 3 months for projects to cut new releases in response to the change. This deadline can be extended if deemed necessary.

Closes #4520; Closes #4519.

Pull Request Checklist

• Changes have tests
• News fragment added in newsfragments/.
  (See documentation for details)

[jaraco]

I've had to move the DeprecationWarning to only trigger on access to setuptools.command.test.test. There are many accesses to attributes of the module by pytest and doctest that are needed to fail, so the DeprecationWarning won't trigger if someone merely imports the module.

[jaraco]

I can confirm that this change allows previously-failing install of cssbeautifier to succeed:

 🐚 pip-run git+https://github.com/pypa/setuptools@refs/pull/4522/head -- -m pip-run --no-build-isolation --no-binary cssbeautifier --no-cache --no-deps -v cssbeautifier -- -c pass
  WARNING: Did not find branch or tag 'refs/pull/4522/head', assuming revision or ref.
Collecting cssbeautifier
  Downloading cssbeautifier-1.15.1.tar.gz (25 kB)
  Preparing metadata (pyproject.toml) ... done
Building wheels for collected packages: cssbeautifier
  Building wheel for cssbeautifier (pyproject.toml) ... done
  Created wheel for cssbeautifier: filename=cssbeautifier-1.15.1-py3-none-any.whl size=30528 sha256=1d80e111c61927bf5da0f8c769aa05006bc0a5f289c9a6c84756a9e98fc6592b
  Stored in directory: /private/var/folders/f2/2plv6q2n7l932m2x004jlw340000gn/T/pip-ephem-wheel-cache-obs8yfgq/wheels/c8/5a/e4/c2ac980978446ee433fb66088f06b3d35bd66f25bc3e88bd52
Successfully built cssbeautifier
Installing collected packages: cssbeautifier
Successfully installed cssbeautifier-1.15.1

[notice] A new release of pip is available: 24.1.2 -> 24.2
[notice] To update, run: pip3.13 install --upgrade pip