channeldb: update TestQueryPayments to cover duplicate payments
channeldb: update TestQueryPayments to cover duplicate payments

Update our current tests to include lookup of duplicate payments. We
do so in preparation for changing our lookup to be based on a new
payments index. We add an append duplicate function which will add a
duplicate payment with the minimum information required to successfully
read it from disk in tests.

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