lnwallet/interface_test: add test to detect confirmation of change output spend tx
lnwallet/interface_test: add test to detect confirmation of change output spend tx

In this commit, we add a new test to the existing set of wallet tests to
ensure we can properly detect the confirmation of transactions that
spend our change outputs. We do this as a measure to prevent future
regressions from happening where the wallet doesn't request its backend
to be notified of when an on-chain transaction pays to a change address,
like with the recently discovered SendOutputs bug.

As is, this test will not pass until we update the btcwallet dependency
in the next commit.

aakselrod/lnd,LightningNetwork/lnd,aakselrod/lnd,LightningNetwork/lnd