Memoize ActiveRecord::Migrator.migrations in tests

What does this MR do?

Monkey-patches ActiveRecord so that the list of migrations is only read once in a test run

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

This is free time off every spec with no downside (that I can see). Saves 0.2-0.5 seconds per spec in my local testing (which I wouldn't overly trust).

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Spawned off #23034 (closed)