This commit constructs a helper closure assertAmountSent that can be
reused by other functions. The closure returns an error so that it can
be used with wait.NoError or the new wait.InvariantNoError. The latter
is added since the predicate could otherwise pass immediately for the
sphinx_replay_persistence tests, but change shortly after. It also
rounds out the wait package so that we offer all combinations of
predicate and no-error style waits.
