ci: add test coverage to the pipeline artifacts

Collect test coverage reports from the pipeline jobs and make them available through the artifacts, similarly as to how it is done in OST.

Collect the reports, and upload the using the https://github.com/actions/upload-artifact to the [ci.yml](https://github.com/oVirt/vdsm/blob/cc4ba97e3aacef2333c5ef7ce44c7a507e79ae44/.github/workflows/ci.yml#L23) file.