Jenkins should report test results to Gitlab MR

When Jenkins runs the test suite for a branch for which there is an open MR, it should, ideally:

• find the appropriate MR. Using the gitlab API, it shouldn't be hard to find that: GET /merge_requests?source_branch=123-fix-this-and-that
• find if the Jenkins user itself has already reported a comment (Discussion, in gitlab API) on that MR. If so, edit it. Otherwise, add a new one
• Write useful information to it:
  • a link to the right Jenkins page
  • results of the last suite run: how many failures?
  • some aggregated analysis. For example "no test run was all-green, but every scenario has been all-green at least once".
• Flag the thread as resolved/unresolved appropriately, so that the reviewer can't just ignore this data

This seems to be actually better than using Gitlab CI Pipelines, even if that is counter-intuitive. In fact:

• a pipeline doesn't seem to be fit for including new test runs. In fact, we re-test a branch once a day, plus any other run that the developer might trigger
• reaching the data from gitlab pipeline requires many more clicks, and lookig at a complex report from there is not great
• we expect the API for editing a note to be much easier