This is best as be a test in the reader package which tests parse_legacy_drawing and possibly with a second one in the writer package. We might also want to check that there aren't conflicts with comments because I think they use legacyDrawing tags as well.
End-to-end tests can only be supplementary to unit tests. Unit-tests make understanding the intention behind the code a lot easier. I think we do need to need check for the presence of wb.vba_archive because otherwise a relation without a file will be preserved and the resulting file will probably be invalid as a result.
Sheets that contain macros and comments are likely to cause problems. We have a similar situation with hyperlinks and images because we don't model the relations properly. :-/