test: add assertion within integration tests for proper preimage after send
test: add assertion within integration tests for proper preimage after send

In this commit we modify the testSingleHopInvoice test to ensure that
we obtain the proper preimage from the SendPayment RPC upon successful
payment completion.

aakselrod/lnd,aakselrod/lnd,AndrewSamokhvalov/lnd,LightningNetwork/lnd,LightningNetwork/lnd,AndrewSamokhvalov/lnd,ddustin/lnd,ddustin/lnd