Decide how to test the librustzcash FFI

[str4d]

Once #41 is completed, the librustzcash crate should just be an FFI wrapper around a set of pure-Rust APIs. Currently, we test (some of) this FFI indirectly via testing of zcashd. We should instead have FFI tests in the librustzcash crate.

The simple way would be to duplicate tests from the pure-Rust crates, and modify them to use the FFI methods. This would probably be rather hard to maintain. It would be great to figure out a strategy to reduce test duplication, while keeping the individual tests readable. Or we might decide that doing so isn't feasible, and just decide to duplicate test logic, in which case this is an easy issue to close 😄

[str4d]

We moved the FFI into https://github.com/zcash/zcash, so this issue is now obsolete. If we ever implement a general-purpose FFI, we can figure out testing of it then.