tests: check for support of custom fields

[jku]

• spec requires support custom fields anywhere in the json :

  Implementers who encounter undefined attribute-value pairs in the format must include the data when calculating hashes or verifying signatures and must preserve the data when re-serializing

• we should test this. python-tuf has a unrecognized_fields in (all?) it's objects that can be used for this
• Mostly the test is interesting if the custom fields are withing "signed" (to ensure hash calculation contains these fields)

[jku]

cc @rdimitrov : you mentioned this case in community meeting. Please add notes if you had specific things in mind