Fixes#976. Joinmarket will allow, but warn, when
non-zero change values (which are not included in
the transaction) result from sweeps (due to the
impossibility of knowing the exact value in advance).
However, prior to this commit, if that estimation
inaccuracy resulted in a negative change value that
was to be ignored, there was a crash due to the fact
that `jmbitcoin.amount_to_str` does not allow negative
values. Hence we instead print the number in the warning
without going through this formatting function.
Adds tests case for negative sweep change.
Adam Gibson