Missing documentation for `tests.toml` file in practice exercise docs

The documentation on building practice exercises doesn’t seem to mention the .meta/tests.toml file at all, even though it’s required for practice exercises for which a problem exists in problem-specifications. I think this file should at least be mentioned here.

Tagging @ErikSchierboom as requested by @iHiD

This is technically not entirely true, as the website doesn’t care for tests.toml at all. It is mostly there at the moment for the benefit of track maintainers, although we could do some cool things with it once we find some time (e.g. sending updates to tracks that a new test case was added or something).

I’ll write up some docs.

Documentation has been added in Document tests.toml by ErikSchierboom · Pull Request #465 · exercism/docs · GitHub.

See Practice Exercises | Exercism's Docs for the updated documentation

Great, thanks! :pray: