[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/wallet-encryption.py

@@ -31,7 +31,7 @@ class WalletEncryptionTest(BitcoinTestFramework):
 
         # Encrypt the wallet
         self.nodes[0].node_encrypt_wallet(passphrase)
-        self.nodes[0] = self.start_node(0, self.options.tmpdir)
+        self.start_node(0)
 
         # Test that the wallet is encrypted
         assert_raises_jsonrpc(-13, "Please enter the wallet passphrase with walletpassphrase first", self.nodes[0].dumpprivkey, address)