E2E new discussion does not drop MR from merge trains

What does this MR do?

New E2E test for case gitlab-org/quality/testcases#1010 (closed) : New thread discussion does not drop MR from merge trains results in MR will be merged successfully.

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content