Find test issues by hash

What does this MR do and why?

This consolidate the strategy to find issues in lib/gitlab_quality/test_tooling/report/relate_failure_issue.rb and lib/gitlab_quality/test_tooling/report/slow_test_issue.rb to use #find_issues_by_hash, which is already used successfully in lib/gitlab_quality/test_tooling/report/flaky_test_issue.rb.

This also changes the search behavior to filter on specific labels so that we don't touch separate issues for now. For instance, a failureflaky-test issue wouldn't update the reports in a "test failure" or "slow test" issue.

Later, we'll move the reports to notes instead of issue description so that we can consolidate issues into a single "test issue".

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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.

Closes #51 (closed)