[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/blockchain.py

@@ -146,7 +146,7 @@ class BlockchainTest(BitcoinTestFramework):
             pass  # The node already shut down before response
         self.log.debug('Node should stop at this height...')
         self.nodes[0].process.wait(timeout=BITCOIND_PROC_WAIT_TIMEOUT)
-        self.nodes[0] = self.start_node(0, self.options.tmpdir)
+        self.start_node(0)
         assert_equal(self.nodes[0].getblockcount(), 207)