Update e2e testcase links to gitlab project

What does this MR do and why?

Updates all the e2e hardcoded testcase links to their new home in the gitlab project.

Also updates the testing guide to reflect moving test cases to the gitlab project.

Step 3 of https://gitlab.com/gitlab-org/quality/team-tasks/-/issues/1079.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.