Add some sort of link between cython and tests

- Closes #14.

    ``add_pytest`` accepts *.pyx files and compiles them. However, pytests can
    only discover pure python files. So the compiled extensions should be
    imported in a pure python test file.

- Closes #13

    Tests (and other files) are added via add_python_module, so anything that
    works for add_python_module should work for tests. This includes
    configurable tests/files, as well as cython extensions. However, only
    discoverable test files ("tests?_.*\\.py") are added to the ctest
    framework.