[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/keypool.py

@@ -19,7 +19,7 @@ class KeyPoolTest(BitcoinTestFramework):
         # Encrypt wallet and wait to terminate
         nodes[0].node_encrypt_wallet('test')
         # Restart node 0
-        nodes[0] = self.start_node(0, self.options.tmpdir)
+        self.start_node(0)
         # Keep creating keys
         addr = nodes[0].getnewaddress()
         addr_data = nodes[0].validateaddress(addr)