[bitcoin/bitcoin] Add tests for RPC help (#14020)

I am not an expert about functional test, and their implementations; but I like to learn new things, that’s why I am asking (maybe stupid) question.

In this kind of functional test, wouldn’t be easier simply to check the sha256 output?

«`
$ ./src/bitcoin-cli help | sha256
2ae3a94e05bbb98323f72f19272bde01f075db16c7e4357bc46815a1393cd44a
«`

thanks :-)