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.
Rewrite the BIP66 functional tests to reflect height-based activation,
and move it out of the extended test suite.
Remove the unnecessary bipdersig.py test
