CI: Add label test to ensure lfns are left intact

Since LFNs are stored in FAT using the VOLID bit (amongst others), we
need to check that:
  when creating a label that we don't error on their existence.
  when deleting a label that we don't destroy or corrupt them.

Note:
   MS-DOS 6.22 and DR-DOS 7.01 fail to create a label if there are LFNs
existing, which is presumably due to their lack of LFN support.