gitlab-shell needs end-to-end tests of bin/gitlab-shell being called, it calling APIs, and then calling custom hooks

@nick.thomas @ashmckenzie : Makes sense with all the relevant issues / MRs to clean this up now closed, to have an issue to track how we recover from this and re-merge some version of !214 (merged).

As noted in #147 (comment 93464123) !214 (merged) (since reverted in !228 (merged)) introduced some regressions to the newly added SSH certificate support in 11.2.

As "Work on improving the test coverage for gitlab-shell..." in the linked comment says, this got past review because there's no end-to-end testing of invoking gitlab-shell, it then invoking some APIs, and finally invoking some custom hooks and passing the right GL_ID to them in the various modes (key, user, username).

There should also be a test in there somewhere to test for how invalid API responses are handled. See cce62b35