Expand merge request settings in the failing E2E spec

What does this MR do?

Quick fix for new_discussion_not_dropping_merge_trains_mr_spec. The error appeared because of the change in Settings behaviour when clicking "Save changes" in "Merge requests" section. Previously when enable_merge_train was executed it left "Merge requests" section uncollapsed and so the spec could click on "All discussions must be resolved" checkbox.

Now when enable_merge_train runs and clicks "Save changes" "Merge requests" section is being collapsed and we need to expand it once more before enable_merge_if_all_disscussions_are_resolved.

• Failure issue: #323421 (closed)

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
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• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content