test: remove concurrent payment sends from testMultiHopPayments
test: remove concurrent payment sends from testMultiHopPayments

testMultiHopPayments was failing intermittently due to a lack of
thread-safety in the gRPC stream. This commit takes payment sending out
of goroutines so that they execute serially.

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