Immature coinbase spends are allowed in the memory pool if they can be mined in the next block.
They are not allowed in the memory pool if they cannot be mined in the next block.
This regression test tests those edge cases.
Ported txnmall.sh to Python, and updated to match
recent transaction malleability changes.
I also modified it so it tests both double-spending
confirmed and unconfirmed (only-in-mempool) transactions.
Renamed to txn_doublespend, since that is really what is
being tested. And told the pull-tester to run both
variations on this test.
Jonas Schnelli