extras: ci: enable coverage in merge requests

This enable coverage report in the diff panel for the MR:

Having the coverage shows when we are sure some changes are not tested, which is helpful to assert whether a change is dangerous or whether a new test misses a part of the change from the same MR.

It also adds coverage result to detect when a test is disabled or a whole branch removed from testing, which is helpful since some parts of some tests are loaded through modules, and changes like buildsystem ones might break this silently.