Test snapshot purge retry

Scenario

  1. Create and attach a Longhorn volumes.
  2. Write some data to the volume then create the 1st snapshot. e.g. 
    dd if=/dev/urandom of=/dev/longhorn/<Longhorn volume name> bs=1M count=100
  3. Try to delete the 1st snapshot. The snapshot will be marked as Removed then hidden on the volume detail page.
  4. Write some non-overlapping data to the volume then create the 2nd snapshot. e.g. 
    dd if=/dev/urandom of=/dev/longhorn/<Longhorn volume name> bs=1M count=100 seek=100
  5. Re-try deleting the 1st snapshot via UI.
  6. Verify snapshot purge is triggered:
    1. The 1st snapshot will be coalesced with the 2nd one, which means the size should be 200Mi.
    2. The final snapshot name is the name of the 2nd one.
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