WIP: Enable Test Coverage Visualization in merge requests
This is a follow-up from !3606 (comment 148014).
Test Coverage Visualization feature displays test coverage in merge request.
This MR is built on top of !3606 (merged) and needs to be eventually rebased.
I requested the feature to be enabled in the CI in https://gitlab.isc.org/isc-private/devops/-/issues/123.