Parallelize test jobs via knapsack

Enable knapsack for QA test jobs.

Applies to the 3 slowest jobs with the most tests:

• {ce|ee}-instance
• {ce|ee}-relative_url
• {ce|ee}-update

The rest of the jobs include too few tests to be worthwhile using knapsack because starting the GitLab container in each job takes a large part of the total runtime. And the slowest job overall, {ce|ee}:kubernetes only has one test.

• See: https://gitlab.com/gitlab-org/gitlab-ce/issues/64953
• CE MR with knapsack report: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/30886
• EE MR with knapsack report: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14755