main: extra interface test for simple named types

testdata/interface.go

@@ -7,6 +7,7 @@ func main() {
 	printItf(5)
 	printItf(byte('x'))
 	printItf("foo")
+	printItf(Foo(18))
 	printItf(*thing)
 	printItf(thing)
 	printItf(Stringer(thing))

testdata/interface.txt

@@ -3,6 +3,7 @@ Thing.Print: foo
 is int: 5
 is byte: 120
 is string: foo
+is Foo: 18
 is Thing: foo
 is *Thing: foo
 is *Thing: foo