adding test for generate_cpp() so it can be refactored safely
adding test for generate_cpp() so it can be refactored safely

stan-dev/stan,stan-dev/stan,stan-dev/stan,stan-dev/math,stan-dev/stan,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/stan,stan-dev/math,stan-dev/math,stan-dev/math