Fix merged train specs flakiness and refactor around merge train and merged results spec

What does this MR do and why?

References

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• To fix failure in https://gitlab.com/gitlab-org/gitlab/-/issues/494653+
• Also, as discussed https://gitlab.com/gitlab-org/gitlab/-/issues/494653#note_2180578226, I have moved merge train spec out of transient bucket. This should allow it to run in all environments. Besides, the old merge train test case within pipelines_for_merged_results_and_merge_trains_spec.rb has been running as :blocking anyways. I think this change is reasonable.
• Add more complexity to merge train test scenario: Have multiple MRs on the same train instead of 1 like before.
• Follow up with that, I removed the merge train test case within the old pipelines_for_merged_results_and_merge_trains_spec.rb
• Merge train is now its own spec, so as merged result.
• Some refactoring around supporting methods for these specs

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Related to #494653