Port/fix txnmall.sh regression test

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.