Fix the flaky test in: spec/features/merge_request/user_sees_diff_spec.rb:79` / `expected to find css ".js-fork-suggestion-button" 1 time but there were no matches`

What does this MR do?

Re-enables the spec in #196749 (closed) - after a lot of testing, I haven't been able to reproduce a failure.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

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

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

#196749 (closed)