sync: disable flaky WaitGroup misuse test in short mode

Update #11443

Change-Id: Icb7ea291a837dcf2799a791a2ba780fd2a5e712b
Reviewed-on: https://go-review.googlesource.com/11721Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>

src/sync/waitgroup_test.go

@@ -69,6 +69,9 @@ func TestWaitGroupMisuse(t *testing.T) {
 
 func TestWaitGroupMisuse2(t *testing.T) {
 	knownRacy(t)
+	if testing.Short() {
+		t.Skip("skipping flaky test in short mode; see issue 11443")
+	}
 	if runtime.NumCPU() <= 2 {
 		t.Skip("NumCPU<=2, skipping: this test requires parallelism")
 	}