lnprototest: API improvement and introduction of test utils function

[vincenzopalazzo]

This PR introduces some code improvements related to lnprototest, in particular now:

• lnprototest is able to print the log to the console in case of failure, and make a backup of the log file in the /tmp dir;
• introduce some utils function in the tests dir related to:
  • test utils, included a function to manage the runner and some others function useful to build the tests. This also introduces a bug fixing to stop the runner when a test by lnprototest fails;
  • spec utils, include a collection of functions to build a well-formed sequence of events to make lightning operations, like connecting to a node or opening a channel. The last change gives the possibility to write tests a smaller