add TestRenterAsyncDownload
add TestRenterAsyncDownload

mharkus/Sia,NebulousLabs/Sia,ericflo/Sia,ChrisSchinnerl/Sia,ChrisSchinnerl/Sia,ericflo/Sia,MrStrong/Sia,mtlynch/Sia,MrStrong/Sia,mharkus/Sia,mtlynch/Sia,NebulousLabs/Sia