Test pageId clashes across fileGrps for cached METS

[kba]

          Note: we should also add test coverage for this. AFAICS, we need a cached `add_file` under the circumstance that a file for that page already exists, but in a different fileGrp.

Originally posted by @bertsky in https://github.com/OCR-D/core/issues/1195#issuecomment-1973882852