test: add test for gccgo issue #15722

Change-Id: I4faf9a55414e217f0c48528efb13ab8fdcd9bb16
Reviewed-on: https://go-review.googlesource.com/24845Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>

test/fixedbugs/issue15722.go

+// errorcheck
+
+// Copyright 2016 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.
+
+// Checks to make sure that the compiler can catch a specific invalid
+// method type expression. NB: gccgo and gc have slightly different
+// error messages, hence the generic test for 'method' and not something
+// more specific.
+
+package issue15722
+
+type T int
+type P *T
+
+func (T) t() {}
+
+func _(p P) {
+	P.t(p) // ERROR "method"
+}