refactor: improve update script multi-class support

Extract class_name from FAIL header in test output instead of relying
on a test_class parameter. This correctly scopes method lookups to
the right class when v2 and v3 share identical method names.

- Add class_name field to TestCount (parsed from test output)
- Scope update_test_file by class_name to avoid cross-class matching
- Add process_class() helper to reduce duplication in main()
- Default to running both v2 and v3 classes when no --test-class given

Author: valentijnscholten