Add automated tests to verify strict NU5 consensus equivalence for subsidy.

[nathan-at-least]

While there are some unit tests that the Subsidy::NU5 calculations match NU5 (current Zcash Mainnet), it would be better to have input->output equivalent test validation from zcashd.

There are a few ways to approach this, but I think my current preferred approach is:

• Tweak zcashd to generate issuance values into a test vector file.
• Add zcashd regression test against that vector.
• Test against that vector here.