Remove casts to concrete collection implementations in tests

[ashmrtn]

Some tests in internal/m365/collection/exchange/backup_test.go like this rely on being able to cast a collection to prefetchCollection. While the tests pass for the moment because the mock returns data in such a way that it ends up with a prefetchCollection these tests should really be updated to just use the data.BackupCollection API

Similar issues are present in at least OneDrive tests (src/internal/collection/drive/collections_test.go) and possibly groups tests