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.
Checks memory before and after a transaction that requires a private key.
Each time, 32 bytes of memory for a private key should be used.
Tested in wallet.py instead of its own file to save testing time.
