Fix places in merge_request_spec.rb that persist changes to the project

What does this MR do?

This MR applies the method in !39996 (diffs, comment 402591440) to other places in merge_request_spec.rb where subject.project is mutated.

This is necessary because of the create_default that is applied to projects within this spec causing the same project to be used whenever a project factory is required. For any parts of this spec where the merge request project is mutated, we need to be careful to create a new project to avoid having this change affect other tests.

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