Enable the ability to have src/include for tests in separate location

I would really like Pika to have `tests/src` and `tests/include` or something similar. So, when a pure user is getting started with input files and Peacock they do not see a whole bunch of objects that are truly not indented for use, but are there for testing purposes. For example, we have a set of Kernels for MMS forcing functions, an end-user should be using or seeing these.
