Clarified transactionpool_test.go's spendCoins()
Clarified transactionpool_test.go's spendCoins()

The comment now reflects that it's a clone of wallet's SpendCoins()

MrStrong/Sia,mharkus/Sia,mtlynch/Sia,ericflo/Sia,NebulousLabs/Sia,ChrisSchinnerl/Sia,mnsl/Sia,mharkus/Sia,mtlynch/Sia,NebulousLabs/Sia,ericflo/Sia,kustomzone/Sia,rivine/rivine,MrStrong/Sia,Mingling94/Sia,johnathanhowell/Sia,rivine/rivine,ChrisSchinnerl/Sia,mnsl/Sia