We were relying heavily on NodeFactory to do some magic before instantiating
the Node with rpc and daemon initialized, that meant that we'd have to replace
all 3 classes when customizing the node to our needs. Moving that
initialization into the node itself means that the LightningNode class now can
be swapped out and customized, without having to wire everything else through.
`DEVELOPER=1` assumes that the binary has been compiled with developer set to
true, which might not be the case for plugin developers. Setting this to 0 by
default has no effect in c-lightning since we always at least set it in
`config.vars` but may prevent some issues outside.
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
