build: fixing problem with --make-args and adding unit tests for running catkin unit tests

When looking at the documentation asked about in #72, I found that `--make-args` was basically broken. I've added a unit test which tries to run some simple googletest unit tests via `catkin build`.
I think it would be more appropriate to return an empty string or None here. We can account for it needing to ultimately be a list in the code below.