Enable Test coverage visualization

Currently we don't have Enable Test coverage visualization enabled. I believe we should enable it in order to notice missing coverage spots better.

Implement test coverage (use zoekt project as a template)

run coverage example: https://gitlab.com/gitlab-org/search-team/gitlab-zoekt-indexer/-/blob/f73b816c2ac39ff3b003eb0cdd36e52e46f0181d/.gitlab-ci.yml#L27-L36