In this commit, an assertion is added to the bolt 3 commitment tx tests
that ensures that the local and remote balances add up to the expected
funding amount. Adding this assertion uncovered a borked test vector
which is also fixed in this commit.
Previously whether or not to add test htlcs was implictly controlled by
a nil value of the HtlcDescs test parameter. With the conversion to
json, that nil value got lost.
The reason that the test still passed is because with the fee rate of
the no-htlc test case, the htlcs were trimmed. Also because in the test
json, balances are specified after applying htlcs, the test didn't fail
with a mismatching balance.