Test backing image download to local

Test step

  1. Create and attach a volume (recommended volume size > 1Gi).
  2. Write some data into the file then calculate the SHA512 checksum of the volume block device.
  3. Create a backing image from the above volume. And wait for the 1st backing image file ready.
  4. Download the backing image to local via UI (Clicking button Download in Operation list of the backing image). => Verify the downloaded file checksum is the same as the volume checksum & the backing image current checksum (when Exported Backing Image Type is raw).
  5. Create and attach the volume with the backing image. Wait for the attachment complete.
  6. Re-download the backing image to local. => Verify the downloaded file checksum still matches.

GitHub Issue: https://github.com/longhorn/longhorn/issues/3155

[Edit]