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.
The best things is to redo it. Split the relevant sheet.xml out of the test Excel file and move it to the reader package. The test will then take this as source and check the assignment.
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. :-/