runtime: skip stack growth test in short mode

LGTM=dvyukov
R=dvyukov
CC=golang-codereviews
https://golang.org/cl/78410043

src/pkg/runtime/stack_test.go

@@ -123,6 +123,9 @@ func TestStackMem(t *testing.T) {
 
 // Test stack growing in different contexts.
 func TestStackGrowth(t *testing.T) {
+	if testing.Short() {
+		t.Skip("skipping test in short mode")
+	}
 	t.Parallel()
 	var wg sync.WaitGroup