Refactor the GitLab system tests

[slarse]

Currently, the GitLab system tests have two big warts.

1. All of the tests exist in a single test module, which is quite large.
2. For some reason, I decided that putting most asserts into a module separate from the tests was a good idea (it's not).

To address 1), the single module can be split according to the command categories (repos, issues, etc).

To address 2), the assertions can be brought back into the modules where they are relevant, and inlined into tests where appropriate. The _assert_on_projects helper could be made a public helper function as it's likely to be needed in every test module.