Functional test opcodes

[ajtowns]

Would be good if the functional tests could automatically handle new opcodes via the binana json stuff. In particular:

• tests for the feature being enabled/disabled (in feature_taproot.py? see CSFS code)
• spenders when enabled in feature_taproot.py?
• test_framework/script.py: OP_FOO=CScriptOp(0xF0) setting, and is_op_success behaviour
• separate data/feature_tx_valid.json files for unit tests? ditto invalid