You can directly expose the partitions using the tool `kpartx`, which will create a `loop` device for you:

  > sudo kpartx -a test.raw
  > lsblk
  NAME      MAJ:MIN RM   SIZE RO TYPE MOUNTPOINT
  loop0       7:0    0     4G  0 loop
  └─loop0p1 253:0    0     4G  0 part
  > sudo kpartx -d test.raw