Move model overrides into own spec location

What does this MR do?

This fixes an auto-loading issue with spring which caused:

MergeRequestWithoutMergeRequestDiff causing superclass mismatch for class

Fixes #258543 (closed)

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