routing: replace sort.Sort with sort.Slice in heap_test.go
routing: replace sort.Sort with sort.Slice in heap_test.go

Use sort.Slice in TestHeapOrdering function in routing/heap_test.go,
as part of the move to use new language features.

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