pyln: Allow users to override the LightningNode class

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.

contrib/pyln-testing/pyln/testing/utils.py

@@ -465,7 +465,7 @@ class ElementsD(BitcoinD):
 class LightningD(TailableProc):
     def __init__(self, lightning_dir, bitcoindproxy, port=9735, random_hsm=False, node_id=0):
         TailableProc.__init__(self, lightning_dir)
-        self.executable = 'lightningd/lightningd'
+        self.executable = 'lightningd'
         self.lightning_dir = lightning_dir
         self.port = port
         self.cmd_prefix = []
@@ -903,7 +903,7 @@ class LightningNode(object):
 class NodeFactory(object):
     """A factory to setup and start `lightningd` daemons.
     """
-    def __init__(self, testname, bitcoind, executor, directory, db_provider):
+    def __init__(self, testname, bitcoind, executor, directory, db_provider, node_cls):
         self.testname = testname
         self.next_id = 1
         self.nodes = []
@@ -912,6 +912,7 @@ class NodeFactory(object):
         self.directory = directory
         self.lock = threading.Lock()
         self.db_provider = db_provider
+        self.node_cls = node_cls
 
     def split_options(self, opts):
         """Split node options from cli options
@@ -985,7 +986,7 @@ class NodeFactory(object):
         # Get the DB backend DSN we should be using for this test and this
         # node.
         db = self.db_provider.get_db(lightning_dir, self.testname, node_id)
-        node = LightningNode(
+        node = self.node_cls(
             node_id, lightning_dir, self.bitcoind, self.executor, db=db,
             port=port, options=options, **kwargs
         )