[CI]: use real world software

[GeopJr]

Describe the request

One of the big issues I have with writing tests for things outside Utils is that we don't have a way to do real world testing.

Aka, if I write a re-implementation of a mastoapi server, we will only be testing the frozen in time mastoapi version. i.e. if I mock /api/v1/instance and it passes but Mastodon changed something from int to float, then the test might pass but Tuba might fail on runtime.

What I'd like to do is spawn the GoToSocial testrig. GTS is the easiest and most lightweight server to run as we can use binaries and in-memory sqlite. Plus the testrig comes with a pre-populated instance.

Obviously I wouldn't want downstreams to ever run it, so it'll be something for this repo only.

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