Move downstream tests to GitHub CI

[jakobnissen]

This makes TS's tests quicker to run locally, and makes it easier to add more downstream tests without bloating the test environment.

Note to reviewers: Let's just see if this runs correctly with CI. If so, we can

• Add more downstream tests. No reason not to add lots, if their test suite is relative short IMO
• Merge this before e.g. Set Random and Test to be weak deps #152 and rebase that onto master. Downstream tests will still fail for Set Random and Test to be weak deps #152 due to compat reasons, but it will clearly show that TS's own tests pass.

[mkitti]

This looks promising.

[mkitti]

What's next here?

[jakobnissen]

I'll add a few more downstream tests, and if they work, merge this. Then I'll rebase #152 on this, and if it's alright with you, merge that and cut a release