Task: Update the tests for the new ETL pipeline (feature-new-etl)

## Update the tests for test_diff_backup, test_vacant_properties and test_ppr_properties for the new ETL pipeline

Some existing tests work as is for the new ETL pipeline but `test_diff_backup.py`, `test_vacant_properties` and `test_ppr_properties` need additional changes.

## Acceptance Criteria

- [ ] DiffReport is tested
- [ ] vacant_properties or sub-components are tested

## Additional context

Previously `vacant_properties` didn't take any arguments, but now it is after `opa_properties` in the pipeline.
We may want to separate out testable sub-functions because the existing tests requires a lot of mocking which makes it less clear what is actually being verified.