test: add inherited interface test to ddd.go
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
http://codereview.appspot.com/5767043

apparentlymart/golang,tv42/old-go,tv42/old-go,apparentlymart/golang,tv42/old-go,gnanderson/go,gnanderson/go,pombredanne/go-deleteme,dsriram/golang,tv42/old-go,dsriram/golang,tv42/old-go,pombredanne/go-deleteme,gnanderson/go,tv42/old-go,gnanderson/go,gnanderson/go,pombredanne/go-deleteme,dsriram/golang,dsriram/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,dsriram/golang,apparentlymart/golang,pombredanne/go-deleteme,apparentlymart/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,gnanderson/go,tv42/old-go,dsriram/golang,dsriram/golang,pombredanne/go-deleteme,gnanderson/go