Integrate test coverage tracking with gitlab CI

If we did a coverage run as part of our CI, and then gave that coverage run to gitlab in cobertura format, supposedly gitlab could track the coverage over time and tell people when their patches were missing test coverage.

At least, so I'm told. 😅

We should double check that grcov works on our gitlab CI runners. (Tarpaulin didn't, when last we tried, due to some kernel compatibility issue.)

Gitlab's documentation may be helpful here.