cmd/go, testing: add TestMain support

Fixes #8202.

LGTM=r, bradfitz
R=r, josharian, bradfitz
CC=golang-codereviews
https://golang.org/cl/148770043

src/testing/testing_test.go

@@ -0,0 +1,18 @@
+// Copyright 2014 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 testing_test
+
+import (
+	"os"
+	"testing"
+)
+
+// This is exactly what a test would do without a TestMain.
+// It's here only so that there is at least one package in the
+// standard library with a TestMain, so that code is executed.
+
+func TestMain(m *testing.M) {
+	os.Exit(m.Run())
+}