Use DANDI IDs and paths in testing suite

[CodyCBakerPhD]

The initial setup uses previous conventions of specifying the static s3_url of each asset; this makes it very difficult to do a backtrace to find the original file on DANDI, unless it was precisely recorded with a comment on the same line

It would probably be easier to specify the six-digit dandiset ID together with the path and have a utility function for finding the s3_url