[tests] TestNode: separate add_node from start_node

Separates the act of creating a TestNode object from starting the node.
The test_framework now keeps track of its list of TestNodes, and test
writers can call start_node() and stop_node() without having to update
the self.nodes list.

Github-Pull: #11121
Rebased-From: 36b6268670

test/functional/test_framework/test_node.py

@@ -65,9 +65,13 @@ class TestNode():
         assert self.rpc_connected and self.rpc is not None, "Error: no RPC connection"
         return self.rpc.__getattr__(*args, **kwargs)
 
-    def start(self):
+    def start(self, extra_args=None, stderr=None):
         """Start the node."""
-        self.process = subprocess.Popen(self.args + self.extra_args, stderr=self.stderr)
+        if extra_args is None:
+            extra_args = self.extra_args
+        if stderr is None:
+            stderr = self.stderr
+        self.process = subprocess.Popen(self.args + extra_args, stderr=stderr)
         self.running = True
         self.log.debug("bitcoind started, waiting for RPC to come up")