test: add test for interfaces with unexported methods.

R=rsc
CC=golang-dev
https://golang.org/cl/4271086

test/interface/private.go

+// $G $D/${F}1.go && errchk $G $D/$F.go
+
+// Copyright 2011 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 "./private1"
+
+type Exported interface {
+	private()
+}
+
+type Implementation struct{}
+
+func (p *Implementation) private() {}
+
+func main() {
+	var x Exported
+	x = new(Implementation)
+	x.private()
+
+	var px p.Exported
+	px = p.X
+
+	px.private()			// ERROR "private"
+
+	px = new(Implementation)	// ERROR "private"
+
+	x = px				// ERROR "private"
+}

test/interface/private1.go

+// true  # used by private.go
+
+// Copyright 2011 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 p
+
+type Exported interface {
+	private()
+}
+
+type Implementation struct{}
+
+func (p *Implementation) private() {}
+
+var X = new(Implementation)
+