Add testing infrastructure

[webknjaz]

• Configure pytest with a few essential plugins. e.g. https://github.com/ansible/pylibssh/blob/830218bf59c10e0a265638943878c3b13d46ca1d/pytest.ini
• Set up tox to run pytest allowing to pass extra args via {posargs}. e.g. https://github.com/ansible/pylibssh/blob/830218bf59c10e0a265638943878c3b13d46ca1d/tox.ini#L11-L43
• Add a very basic test infra. e.g. https://github.com/ansible/pylibssh/blob/830218bf59c10e0a265638943878c3b13d46ca1d/tests/unit/version_test.py#L15-L17
  Pro tip: Start with just one test function having only assert True. This will allow you to check that you've configured all of the other bits correctly. Your goal is to see this test in the report and have it green.
• Add a GHA workflow for running the tests. e.g. https://github.com/ansible/pylibssh/blob/830218bf59c10e0a265638943878c3b13d46ca1d/.github/workflows/build-test-n-publish.yml#L656
  Start small, with one job. Then, extend it to be a matrix across different Python versions (as specified in setup.cfg) and OSs.
• Finally, add a few unit tests for helper functions in the extension.
• Add heavier integration tests too.
• Integrate sphinx.testing.fixtures: https://www.sphinx-doc.org/en/master/extdev/testing.html