Test blake2b circuit

[vlopes11]

Currently we have the blake2b circuit tested only as part of the super circuit integrated tests, where the output is expected to match an entry of the lookup table

If the blake circuit computes the value incorrectly, it means the super circuit will fail as no entry will exist for the lookup table

However, testing the super circuit is very expensive, and it is desirable to have a unit/dedicated test to assert the correctness of the blake2b circuit

One option is to create a test circuit that will accept a witness value that is the output of a hash, call the circuit, and expect it to exist on the lookup table.

Originally posted by @vlopes11 in #56 (comment)