sync/atomic: fix data race in tests

Fixes #2710.

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/5541066

src/pkg/sync/atomic/atomic_test.go

@@ -1037,7 +1037,7 @@ func TestStoreLoadSeqCst32(t *testing.T) {
 				if my != i && his != i {
 					t.Fatalf("store/load are not sequentially consistent: %d/%d (%d)", my, his, i)
 				}
-				ack[me][(i-1)%3] = -1
+				StoreInt32(&ack[me][(i-1)%3], -1)
 			}
 			c <- true
 		}(p)
@@ -1078,7 +1078,7 @@ func TestStoreLoadSeqCst64(t *testing.T) {
 				if my != i && his != i {
 					t.Fatalf("store/load are not sequentially consistent: %d/%d (%d)", my, his, i)
 				}
-				ack[me][(i-1)%3] = -1
+				StoreInt64(&ack[me][(i-1)%3], -1)
 			}
 			c <- true
 		}(p)