We were hardcoding the chainparams->chain_hash which caused the query to
return an empty result. By parametrizing the test we can make it work on
elements.
In the c-lightning tests we have `tests/conftest.py` which annotates test
function with the outcome. If we use pyln-testing outside of the c-lightning
tree we cannot rely on that annotation being there, so we assume it passed.
Quite a few of the things in the LightningNode class are tailored to their use
in the c-lightning tests, so I decided to split those customizations out into
a sub-class, and adding one more fixture that just serves the class. This
allows us to override the LightningNode implementation in our own tests, while
still having sane defaults for other users.
We'll rewrite the tests to use this infrastructure in the next commit.
Changelog-Added: The new `pyln-testing` package now contains the testing infrastructure so it can be reused to test against c-lighting in external projects
