Like the DOS 3.3 and ProDOS sector order issues, this change fixes the
physical order of the sectors on 13-sector disks when nibblized.
This change also adds tests for the 13-sector format to verify the
sector order.
One of the crazy things is that _Beneath Apple DOS_ failed me in this
instance because it doesn't discuss what happens to the last byte in
"5 and 3" encoding anywhere (AFAICT). I went back to the DOS 3.1
source released by the Computer History Museum here:
https://computerhistory.org/blog/apple-ii-dos-source-code/
The code is in `appdos31.lst` in the `POSTNIB` routine on line 4777.
