runtime: increase timeout in TestStackGrowth

It looks like maybe on slower builders 4 seconds is not enough.
Trying to get rid of the flaky failures.

TBR=iant
CC=golang-codereviews
https://golang.org/cl/86870044

src/pkg/runtime/stack_test.go

@@ -162,7 +162,7 @@ func TestStackGrowth(t *testing.T) {
 		GC()
 		select {
 		case <-done:
-		case <-time.After(4 * time.Second):
+		case <-time.After(20 * time.Second):
 			t.Fatal("finalizer did not run")
 		}
 	}()