Consider splitting out core/app and node test-infra code

[evan-forbes]

It is currently difficult to use a version of the application that is not yet supported by node because this repo combines the testing helpers/logic for both here. When trying to do so, I effectively had to either go update node, or temporarily remove all of the node code to get testground to build the relevant images for the validator only tests.

[Bidon15]

I guess it's time for a composition interop to get this vicious cycle out of the equation