runtime/race: mark failing test as such.

Fixes race builder.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/8055045

src/pkg/runtime/race/testdata/mop_test.go

@@ -1477,7 +1477,7 @@ func TestRaceFailingSliceStruct(t *testing.T) {
 	<-c
 }
 
-func TestRaceAppendSliceStruct(t *testing.T) {
+func TestRaceFailingAppendSliceStruct(t *testing.T) {
 	type X struct {
 		x, y int
 	}