Almost all test scripts currently need to override the __init__()
method. When they do that they need to call into super().__init__() as
the base class does some generic initialization.
This commit makes the base class __init__() call into set_test_params()
method. Individual test cases can override set_test_params() to setup
their test parameters.
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.
bip9-sofforks.py stop-starts the bitcoind node twice during the test
run, but it doesn't wait for the connection from mininode to open before
continuing with the test. This leads to race conditions where the test
can fail getblocktemplate() because it has no p2p connections.
