Fix broken testing links in CONTRIBUTING.md

Updates broken links in CONTRIBUTING.md

Two tests links were broken in CONTRIBUTING.md file. I replaced them with one pointing to the testing guide and the second with instructions on how to run tests in gitlab pages repository

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