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.
Instead of using ismine to check whether an address can be spent by us,
make the witness version of the script or address first and then use
ProduceSignature with the DummySignatureCreator to check if we can
solve for the script.
Also fixes test cases to reflect this change.
