Add tests for Racker::Processor

Adds tests for Racker::Processor class.

I don't usually write spec tests, but hopefully I haven't bastardized anything too badly. Some of the tests are more on the integration end of the spectrum, but that seemed like the least brittle way to go until `#execute!` can be refactored. Also introduces a couple of fixtures that may be less necessary after some refactoring, but I think are needed for the moment.