[tests] better functional and integration tests

[wgtdkp]

Current functional tests are done with OTBR and openthread written in shell script. The scripts grow complicate when we need fine-granted checking (for example, verify the returned data). We need to refactor the test scripts in Python, just as we do for openthread.

On the other hand, there are features cannot be easily tested with OTBR and openthread, for example, failure cases which cannot be triggered through OTBR or openthread API. What we do for OT Registrar is to write simulated Thread Joiner for testing, and it could also be useful to write simulated border agent and leader for OT Commissioner.