add documentation for using treq.testing to test klein applications

[betinacosta]

With Flask you can do this:

class TestApp:
    @pytest.fixture
    def client(self):
        flask_app = app
        with flask_app.app.test_client() as client:
            yield client

But Klein app does not have a test_client or anything like it. Is there a way to test it?

[glyph]

You can use treq.testing for this, which is documented here: https://treq.readthedocs.io/en/release-22.1.0/testing.html#writing-tests-for-twisted-web-resources

While trying to answer this question, I was a bit surprised to find that there's nothing in Klein's docs that demonstrate putting this all together! I am going to repurpose this issue for the documentation improvement that suggests this approach.