Propagate Warnings and Test

This PR adds partial support for pushing CSM warnings into Python. With the addition of a csm::warning, it is necessary to have the swig wrapper be able to push warnings through to the wrapping languages. This is a minor code change to the .i files.

In order to test this, it is necessary to load a plugin dynamically that can be forced to throw an error. Therefore, this PR also adds a stripped down implementation (a fixture basically) that is used solely to test the interface. The bulk of the code changes are writing this fixture and adding the ability to CMake to load the fixture for testing.