runtime: fix race in TestChanSendBarrier

Fixes race detector build.

Change-Id: I8bdc78d57487580e6b5b8c415df4653a1ba69e37
Reviewed-on: https://go-review.googlesource.com/12087Reviewed-by: Austin Clements <austin@google.com>

src/runtime/chanbarrier_test.go

@@ -54,6 +54,7 @@ func TestChanSendBarrier(t *testing.T) {
 
 func testChanSendBarrier(useSelect bool) {
 	var wg sync.WaitGroup
+	var globalMu sync.Mutex
 	outer := 100
 	inner := 100000
 	if testing.Short() {
@@ -73,7 +74,9 @@ func testChanSendBarrier(useSelect bool) {
 				}
 				garbage = make([]byte, 1<<10)
 			}
+			globalMu.Lock()
 			global = garbage
+			globalMu.Unlock()
 		}()
 	}
 	wg.Wait()