Add tests for hashes/length in metafiles

[jku]

RepositorySimulator by default does not include hashes/length in METAFILE (so within timestamp and snapshot metadata).

We should have tests with them:

• we should at a minimum have a simple test that does two ordinary refreshes against a repo that uses hashes/length ( and maybe against repos with only hashes or only length but this is less important): this verifies that client actually supports the feature
• then could add tests where hashes/length data are set to wrong values to verify client actually uses the hashes/length
• test_snapshot_rollback_with_local_snapshot_hash_mismatch() (RepositorySimulator._compute_hashes_and_length is broken #128) already has a very specific test about targets rollback -- it is currently broken though
• possibly we could make all rollback tests parametrized so that they get run against repo without hashes/length as well as against a repo with hashes/length