Soundness tests

[lorenzb]

We should test that invalid proofs are rejected by the verifier.
Suggested approach:

1. Manually create a few invalid proofs
2. Test edge cases (e.g. empty nodes list in proofBlob)
3. Mutate valid proofs in randomly chosen single byte

[relyt29]

proof fuzzing?

[lorenzb]

In the long run that would be nice.
For now, a few basic tests would already mark a substantial improvement.