BullyGB is a fully modular Gameboy testing framework allowing for easy creation of hardware tests. Additionally, it comes with a load of test cases, ranging from easy-to-pass to very intricate and special-edge-case behavior.
Sure! Everything you need to know is written in the src/tests.asm file. An explicit contributing guidelines file will be added soon(tm).
Have a look at the Wiki where all tests are listed and documented.