fix(merkledag/test) panic!

https://travis-ci.org/jbenet/go-ipfs/jobs/45000756

cc @whyrusleeping @jbenet

lol this is starting to happen pretty often

merkledag_test.go

@@ -159,5 +159,5 @@ func runBatchFetchTest(t *testing.T, root *Node) {
 		}(i)
 	}
 
-	wg.Done()
+	wg.Wait()
 }