add tests for force delete snapshot:

- test_snapshot_force_delete_when_snapshot_is_creating
- test_snapshot_force_delete_when_snapshot_is_deleting
- test_snapshot_force_delete_when_snapshot_is_error
- test_snapshot_force_delete_when_snapshot_is_error_deleting

Change-Id: If03e21ee5b617476aa84f6512d2a6346194c9fcb
Merge "add tests for force delete snapshot:"