misc/cgo/test: check API compatibility for cgo pseudo-functions

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/9826043

misc/cgo/test/api.go

+// Copyright 2013 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.
+
+// API Compatibility Checks for cgo
+
+package cgotest
+
+// #include <stdlib.h>
+// const char *api_hello = "hello!";
+import "C"
+import "unsafe"
+
+func testAPI() {
+	var cs *C.char
+	cs = C.CString("hello")
+	defer C.free(unsafe.Pointer(cs))
+	var s string
+	s = C.GoString((*C.char)(C.api_hello))
+	s = C.GoStringN((*C.char)(C.api_hello), C.int(6))
+	var b []byte
+	b = C.GoBytes(unsafe.Pointer(C.api_hello), C.int(6))
+	_, _ = s, b
+}