Move Cascading specific tests out

* Remove all Cascading specific testing from `api_test.clj` and
`api_secondary_test.clj` and put it in `cascading_api_test.clj`.
I understand traps are Cascading specific, but I think they offer some nice functionality that could be added to the MemoryPlatform which isn't really Cascading specific -- they allow you to capture exceptions midway in your query and not break the entire execution.  

I'm happy with moving this test out of the way for the time, and then bringing traps back as a feature to the MemoryPlatform.