ci: add unit-test task and avoid building the binary

What does this MR do?

The tests jobs only run unit tests and don't need the binary (unlike acceptance tests) but the make task depends on gitlab-pages which builds the binary, wasting CI minutes.

This MR adds the unit-test make task which doesn't build the binary and run unit tests.

This is part of an effort to reduce CI minutes consumption in GitLab products in response to the recent change in CI/CD minutes usage for contributors.

TODO

• I added the Changelog trailer (e.g. Changelog: added) to the commits that need to be included in the changelog
• I added unit tests or they are not required
• I added acceptance tests or they are not required
• I added documentation (or it's not required)
• I followed code review guidelines
• I followed Go Style guidelines