gc: add test case for issue 1743

Fixes #1743.
(Actually was fixed earlier, but now we have proof.)

R=ken2
CC=golang-dev
https://golang.org/cl/5649064

test/fixedbugs/bug414.dir/main.go

+// Copyright 2012 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+	  package main
+	
+	  import "./p1"
+	
+	  type MyObject struct {
+	    p1.Fer
+	  }
+	
+	  func main() {
+	    var b p1.Fer = &p1.Object{}
+	    p1.PrintFer(b)
+	    var c p1.Fer = &MyObject{b}
+	    p1.PrintFer(c)
+	  }

test/fixedbugs/bug414.dir/p1.go

+// Copyright 2012 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+	  package p1
+	
+	  import "fmt"
+	
+	  type Fer interface {
+	    f() string
+	  }
+	
+	  type Object struct {}
+	
+	  func (this *Object) f() string {
+	    return "Object.f"
+	  }
+	
+	  func PrintFer(fer Fer) {
+	    fmt.Sprintln(fer.f())
+	  }

test/fixedbugs/bug414.go

+// $G $D/$F.dir/p1.go && $G $D/$F.dir/main.go && $L main.$A && ./$A.out
+
+// Copyright 2012 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package ignored