Writing tests for interactions with LDK

[ConorOkus]

We should look to create a dedicated section on testing.

Currently, it's difficult to write tests for things like opening channels as it requires a second node.

Some things we might include in this section:

• Testing overview, tradeoffs, recommended testing tools
• Testing recipes, common patterns for testing with LDK
• Testing environments, mocking functions, mocking modules

[devrandom]

You might be interested in doing a system test, such as this one for lnrod: https://gitlab.com/lightning-signer/lnrod/-/blob/master/tests/integration-test.py

Note also the Docker method of launching that, at top level of the repo.