channeldb: add test case for index offset greater than index
channeldb: add test case for index offset greater than index

In our current invoice pagination logic, we would not return any
invoices if our offset index was more than 1 off our last index and we
were paginating backwards. This commit adds a test case for this
behaviour before fixing it in the next commit.

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