test: Check that decoderawtransaction heuristic may fail #19831

Adds an example where the heuristic fails

Concept ACK

Would it make sense to be more explicit about what could go wrong (since it'll always be the case that the heuristic matches either iswitness=False or iswitness=True)?

For example:

    # The heuristic produces an absurd result with a negative output value
    heuristic = self.nodes[0].decoderawtransaction(hexstring=TX_HEURISTIC) 
    assert heuristic['vout'][0]['value'] < 0

    # The heuristic produces the same absurd result as iswitness=False
    non_witness = self.nodes[0].decoderawtransaction(hexstring=TX_HEURISTIC, iswitness=False)
    assert_equal(heuristic['txid'], non_witness['txid'])

    # Specifying iswitness=True produces a reasonable result
    witness = self.nodes[0].decoderawtransaction(hexstring=TX_HEURISTIC, iswitness=True)
    assert witness['vout'][0]['value'] > 0

`assert heuristic['vout'][0]['value'] < 0`

The amount doesn't have to be negative. I've modified the tx, so that the amount is positive.

I'm a bit confused by this. Is this something that should be able to fail, or a bug that needs to be corrected in the heuristic?

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Conflicts

Reviewers, this pull request conflicts with the following ones:

• #22437 (test, refactor: add GetTransaction() coverage, improve rpc_rawtransaction by jonatack)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

@MarcoFalke mind addressing laanwj's question?

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🐙 This pull request conflicts with the target branch and needs rebase.

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Fixed by #20595