Add tests for the Sapling proving and verifying APIs

[str4d]

The Sapling logic for proving and verifying was refactored into the zcash_proofs crate in zcash/librustzcash#30. We should add tests!

The tests themselves should be pretty straightforward. However, we can't assume that the test runner has the mainnet Sapling parameters, so we probably want to generate fake parameters on-the-fly (if this is not too complex or slow to do in a test).

[str4d]

I added the "good first issue" label because this would be a good chance for someone new to learn how parameter generation works. It's definitely a larger chunk of work than other issues with the same label.