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.
Create a combinerawtransaction RPC which accepts a json array of hex raw
transactions to combine them into one transaction. Signrawtransaction is changed
to no longer combine transactions and only accept one transaction at a time.
