test/interface/receiver.go: expand to do dynamic

	versions of static checks in receiver1.go

R=r
https://golang.org/cl/155045

test/interface/receiver.go

@@ -58,6 +58,10 @@ func main() {
 	v.V();
 
 //	p = t;	// ERROR
+	var i interface{} = t;
+	if _, ok := i.(P); ok {
+		panicln("dynamic i.(P) succeeded incorrectly");
+	}
 
 //	println("--struct--");
 	var s S;
@@ -76,6 +80,10 @@ func main() {
 	v.V();
 
 //	p = s;	// ERROR
+	var j interface{} = s;
+	if _, ok := j.(P); ok {
+		panicln("dynamic j.(P) succeeded incorrectly");
+	}
 
 //	println("--struct pointer--");
 	var sp SP;