rollback tests missing targets rollback

[jku]

• client must not accept the version of any targets roles in snapshot to decrease: we don't have tests specifically for this
• test_snapshot_rollback_with_local_snapshot_hash_mismatch() tries but currently fails to test this with hashes/length included in snapshot but we should have the basic rollback test first (as mentioned in Add tests for hashes/length in metafiles #131 maybe we can run all the rollback tests with hashes/length in addition to without them)
• there is another rollback-related test we should have: client should not allow targets roles to be removed from snapshot
  (except when snapshot keys are rotated but I think that's covered byt the fast_forward tests)