feature_opcat.py doesn't test block/consensus enforcement #106

test/functional/feature_opcat.py only tests mempool rejection via sendrawtransaction. It never submits a block containing an invalid OP_CAT transaction to verify consensus enforcement. This is arguably okay with 29.1, as mempool enforcement is checked against both standardness and consensus rules, and gives a different error. Though only arguably, because block acceptance (rejection) is still a different code path. But after 29.3 where we only check against standardness rules, it's not really sufficient.

Missing coverage:

• Taproot OP_CAT with insufficient stack elements — rejected by mempool, never tested in a block
• Taproot OP_CAT exceeding stack element size limit — rejected by mempool, never tested in a block
• Segwit v0 OP_CAT (disabled opcode) — rejected by mempool, never tested in a block

The test has get_block/add_block helpers already, and feature_checktemplateverify.py has a fail_block helper
that could be used as a pattern.