Add E2E test of merge trains

What does this MR do?

Adds a new E2E test to the pipelines for merged results spec file because it uses the the same setup, including a runner (which is not cheap to set up)

Closes gitlab-org/quality/testcases#204 (closed)

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content