test: add inherited interface test to ddd.go

The gccgo compiler incorrectly gave an error for this code.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/5767043

test/ddd.go

@@ -60,6 +60,10 @@ type U struct {
 	*T
 }
 
+type I interface {
+	Sum(...int) int
+}
+
 func main() {
 	if x := sum(1, 2, 3); x != 6 {
 		println("sum 6", x)
@@ -207,6 +211,14 @@ func main() {
 		println("i(=u).Sum", x)
 		panic("fail")
 	}
+	var s struct {
+		I
+	}
+	s.I = &u
+	if x := s.Sum(2, 3, 5, 8); x != 18 {
+		println("s{&u}.Sum", x)
+		panic("fail")
+	}
 	/* TODO(rsc): Enable once nested method expressions work.
 	if x := (*U).Sum(&U{}, 1, 3, 5, 2); x != 11 {
 		println("(*U).Sum", x)