DevUX & docs for contributing node tests within node folders

[jackparmer]

we should have detailed, easy-to-follow instructions for adding python-based tests to node folders like

https://github.com/flojoy-io/nodes/tree/main/TRANSFORMERS/ARITHMETIC/ADD

keep in mind that most folks contributing nodes will be scientists/engineers who have no prior experience writing tests or even writing code outside of Python/MATLAB. so we wouldn't want to encourage JS-based cypress tests, for example