[purchasing] set selected offer on purchase

dagger/purchasing.nim

@@ -81,6 +81,7 @@ proc selectOffer(purchase: Purchase) {.async.} =
       cheapest = some offer
   if cheapest =? cheapest:
     await purchase.market.selectOffer(cheapest.id)
+    purchase.selected = some cheapest
 
 proc run(purchase: Purchase) {.async.} =
   proc onOffer(offer: StorageOffer) =

tests/dagger/testpurchasing.nim

@@ -92,6 +92,7 @@ suite "Purchasing":
     discard await market.offerStorage(offer2)
     clock.set(request.expiry.truncate(int64))
     await purchase.wait()
+    check purchase.selected == some offer2
     check market.selected[0] == offer2.id
 
   test "ignores offers that expired":