Test MASP proof/sig verifications

[grarco]

We should ensure to have at least one test that checks the correctness of the proofs/signatures verification in the masp vp. All the integration tests run with mocked builder/verifier so we cannot rely on them. The last masp e2e test will be removed in #3217. We still have the shielded action e2e tests but I believe it would be better to have a specific one.