[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.

test/functional/listtransactions.py

@@ -20,11 +20,7 @@ class ListTransactionsTest(BitcoinTestFramework):
         super().__init__()
         self.num_nodes = 4
         self.setup_clean_chain = False
-
-    def setup_nodes(self):
-        #This test requires mocktime
         self.enable_mocktime()
-        self.nodes = self.start_nodes(self.num_nodes, self.options.tmpdir)
 
     def run_test(self):
         # Simple send, 0 to 1: