Add recommendation on how to run `tox` in CI

[paddyroddy]

Which Section of the Documentation Needs Improving?

http://github-pages.arc.ucl.ac.uk/python-tooling/pages/testing.html#test-runners

What Can be Improved About This Section

i.e. https://github.com/tox-dev/tox-gh as introduced in #460

How to Improve This Section

No response

[p-j-smith]

If you're using some features of uv (such as some of the alternative dependency sources), you will need to use tox-uv too. One way is to also use astral-sh/setup-uv and uv run:

      - name: Setup uv
        uses: astral-sh/setup-uv@f0ec1fc3b38f5e7cd731bb6ce540c5af426746bb # v6
        with:
          python-version: "3.11"
          enable-cache: true
          cache-dependency-glob: |
            **/pyproject.toml
            **/uv.lock
            **/.tox/**/*

      - name: Test with tox
        run: uv run --with tox-uv tox run