Unit Tests Using Mocking Layer

[braddmiller]

As a lightwalletd developer, I would like a comprehensive testing suite to reliably test interactions with the zcashd RPC without having to run a zcashd node to keep the testing suite self contained.

Acceptance Criteria:

• Lightwalletd library test coverage to 80% (coverage is 53.2% at start of ticket)

[LarryRuane]

Merged #164