Unit testing demo

[lizgzil]

For the have-a-go session:

git clone https://github.com/nestauk/dap_tutorials.git
git checkout testing
cd dap_tutorials/unit_tests
conda create --name unit_tests python=3.10
conda activate unit_tests
pip install -r requirements.txt

In this PR I also set up a github action for running the tests for this repo. Since this repo isn't really a python project (so doesnt have a setup.py or pyproject.toml) some parts of github/workflows/pytest.yaml needed to be commented out.

The test that is run should always pass and has no requirements or dependencies (other than pytest), so shouldn't get in the way.