Break up unit tests into separate files tests

[charleskawczynski]

The unit test file is quite long and, the increased size makes it more prone to leaky tests. It might be nice if we take some time to organize the unit tests a bit and break it into separate files, and perhaps use SafeTestsets.jl

One concrete issue is that #112 implements a function once in the test (since it creates a closure over some variables to feed into ForwardDiff), and another in the docs. If we split up the tests a bit more, we may be able to just include shared files between the test and docs directory without duplication. I didn't want to include all of this re-organization in #112, however.