test: Fix too small ext2 block count

The amd64-separate-usr autopkgtest fails always in a Debian unstable schroot environment (with overlayfs) with:

genext2fs: couldn't allocate a block (no free space)

This is caused by specifying a too small amount of blocks:

$ mkdir /tmp/empty
$ genext2fs -b 20 -N 12 -U -d /tmp/empty /tmp/empty.raw
genext2fs: couldn't allocate a block (no free space)
$ genext2fs -b 25 -N 12 -U -d /tmp/empty /tmp/empty.raw
copying from directory /tmp/empty

Increase the minimum to 28 blocks solved the failure.