Rationalise unit test workflows #1741

I don't like that we have two workflows for unit tests:

1. https://github.com/pints-team/pints/blob/main/.github/workflows/unit-test-os-coverage.yml
2. https://github.com/pints-team/pints/blob/main/.github/workflows/unit-test-python-coverage.yml

(I do realise that this was originally my design choice!)

Let's rationalise into a single workflow, ensuring we have coverage on all modern platforms and python versions, including expanding our testing to arm64 architectures.