Fix getbalance handling of spent, unconfirmed change

First: txnmall.sh is the start of an integration test for wallet handling of
mutated (malleable) transaction handling.

Second is a one-line change to IsConfirmed that adds a condition when
considering change outputs : only consider them confirmed if they are
in the memory pool (inspired by a comment sipa made yesterday).

This makes getbalance, getbalance '*', and listunspent all agree when run on the txnmall.sh test wallets. Before this fix, those three would return three different results.

Before:
 listunspent: one 49BTC output
 getbalance: 96 BTC (change counted twice)
 getbalance '*': 46 BTC (spends counted twice)

After: all agree, 49 BTC available to spend.