feat: add self hosted gitlab to integration tests

[cafkafk]

In f94d4d5, I introduced some integration tests that ensure the most important endpoint is tested. However, due to being unable to find a self hosted gitlab I could register on, I haven't set up testing for that.

[algernon]

Regarding testing: how do you feel about mocking the APIs of the different forges with something like mockito, and running tests against those? At least for the self-hostable forges, this would allow testing without access to the outside network, and without having to register and host a test project on either of them. The downside, of course, is that the mocked APIs would have to be faithful enough for the tests to make sense, and would require a bit of extra maintenance.

[cafkafk]

I thought if we went in that direction, we'd go for the nixpkgs style nixos tests, then we wouldn't even need to mock the self-hostable forges, which would be very cool (and allow us to run the tests inside the flake check). The only thing that has stopped me from doing that is I'm not really very familiar with nixos tests, and those integration tests I made just now are just a temporary solution we can keep around until something better.