[Zoe] Add tests for Zoe contracts to use non-fungible assets (setStr and set mathHelpers)

[katelynsills]

Currently all of our contracts (apart from autoswap/uniswap, because the constant product formula only makes sense for fungible tokens with natural number amounts) should work with non-fungible tokens. We need to create test cases to show how the non-fungible tokens work in these contracts with mathHelpers setStr and set.