E2E: Extract knapsack runner

What does this MR do and why?

Describe in detail what your merge request does and why.

• Extracts knapsack runner class so it can be reused in bin/qa runner and in rake task.
• Uses rspec::core runner to not spawn new ruby process

Follow up to !86227 (merged)

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.

Part of: https://gitlab.com/gitlab-org/quality/team-tasks/-/issues/1034