[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/create_cache.py

@@ -18,7 +18,6 @@ class CreateCache(BitcoinTestFramework):
 
         # Test network and test nodes are not required:
         self.num_nodes = 0
-        self.nodes = []
 
     def setup_network(self):
         pass