[rebranch] Update test broken in merge
[rebranch] Update test broken in merge

`async-args.swift` was updated in
29fd8d33446ee1135fb6017b96866c79ed6ed152 but the update looks to have
been lost in a merge. Re-apply the missing portion.

roambotics/swift,benlangmuir/swift,benlangmuir/swift,benlangmuir/swift,glessard/swift,roambotics/swift,glessard/swift,roambotics/swift,apple/swift,apple/swift,roambotics/swift,glessard/swift,benlangmuir/swift,apple/swift,apple/swift,roambotics/swift,glessard/swift,apple/swift,glessard/swift,glessard/swift,benlangmuir/swift,apple/swift,benlangmuir/swift,roambotics/swift