test: add test that caused gccgo undefined symbol error

Change-Id: I94aa87fe951701413b479c05b0bc8810255eb01c
Reviewed-on: https://go-review.googlesource.com/45085
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

test/fixedbugs/bug502.go

+// build
+
+// Copyright 2017 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.
+
+// Linking this with gccgo got an undefined symbol reference,
+// because the private method in testing.TB led gccgo to assume that
+// the interface method table would be defined in the testing package.
+
+package main
+
+import "testing"
+
+type I interface {
+	testing.TB
+	Parallel()
+}
+
+func F(i I) {
+	i.Log("F")
+}
+
+var t testing.T
+
+func main() {
+	F(&t)
+}