Improve Pelias parsing tests layer

[msmollin]

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We currently have a spartan parsing layer in the https://github.com/mapzen/ios/blob/master/src/PeliasMapkitExtensions.swift#L123

It's functional but I think we could improve upon it, especially in the testing layer. Given Swift 4 recently merged in support for multi-line String support, it should make JSON strings far easier to work with for building mock JSON fixtures in the future. But for now we could add some better tests just in general.

[msmollin]

https://github.com/pelias/pelias-ios-sdk/issues/78