Mock App function circuits in Goblin recursion tests

[ledwards2225]

The real kernel circuit will perform two recursive verifications (eventually, folding verifications). Currently we don't have a mocked app circuit at all in the goblin recursion tests so we simply construct two recursive verifications of the previous kernel proof as a stand in approximation. We need to actually construct an app proof since that will be a significant computation. For our purposes the app proof can simply be a circuit of given size comprised entirely of arithmetic gates.

[codygunton]

Closed by AztecProtocol/aztec-packages#4242