fix: remove `test` command and related code from `setup.py`

[pyrooka]

The test command and related function were deprecated for 5 years and it seems the time has come: setuptools maintainers removed it yesterday and that move has broken a bunch of Python packages including this. After looking into several documentations and our source code, I think the right thing to do - instead of making a workaround - is completely removing it from our setup.py file as it hasn't been used for a long time - correct me if I am wrong here and missed something!! Some details:

• Issue in the setuptools repo: https://github.com/pypa/setuptools/issues/4519
• The commit that introduced this: https://github.com/pypa/setuptools/commit/4c0b9f3ee6ee47c597572655567f215c08c90137
• Release note: https://setuptools.pypa.io/en/stable/history.html#deprecations-and-removals

  The test command has been removed. Users relying on 'setup.py test' will need to migrate to another test runner or pin setuptools before this version.

• Extra: https://docs.pytest.org/en/latest/explanation/goodpractices.html#do-not-run-via-setuptools

  Integration with setuptools is not recommended, i.e. you should not be using python setup.py test or pytest-runner, and may stop working in the future.

[ibm-devx-sdk]

:tada: This PR is included in version 3.20.4 :tada:

The release is available on GitHub release

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