tests/auto-prune: Don't go negative

tests/auto-prune: Don't go negative

- First I kept wondering what the magic of 10000 was here before
  looking above and noticing it matched the number of dtb files.
  Make a shared variable so the connection is more obvious
- Next, I *believe* the intention of this test was to test
  the edge case of bytes vs blocks, but we ended up subtracting
  blocks and I think recent FCOS images happened to get small
  enough that we started going negative here.
  Fix this to convert the bytes into blocks.