Tests should test properly signed but invalid Verifiable Credentials

[msporny]

Most of the tests modify a properly signed data structure, which means that verifiers will (properly) reject the signature because its invalid. What we want instead is for the verifiers to reject the data structure because the data structure is invalid. In each test below here, we should modify the property under test to an invalid value and then properly sign it, and then send it to the verifier. The verifier should detect a valid signature, but fail the item because of the invalid property. We should also keep proper positive and negative tests for a valid signature.

"We need to be checking two classes of failure. One - this VC which has a valid signature does not conform to the VC data model, and two - this VC that conforms to the data model has an invalid signature." -- @brownoxford