Fix/adding tests to k8s prerequisits branch

[boekhorstb1]

• added unittests: note that these only run with pytest currently. Try: .venv/bin/python -m pytest tests/modules/test_k8s_prerequisites_check.py
• when running with unittest there are various import errors. Try: /app/rookify/.venv/bin/python -m unittest /app/rookify/src/tests/modules/test_k8s_prerequisites_check.py, also note that these import errors do not occure when testing ExampleHandler module, yet import is done in the same way. Try: /app/rookify/.venv/bin/python -m unittest /app/rookify/src/tests/modules/test_example.py
• because the ModuleHandler loads all the module, I found that I had to mock the class TestK8sPrerequisitesCheckHandler itself, thereby keping the code for preflight method the same... maybe you know another way to mock the ModuleHandler before it gets loaded?