tests/integration: Turn into valid, empty Go package.

This describes the package in a way that can be seen via godoc,
and makes it so that the following command succeeds without
"no buildable Go source files" error:

	go build github.com/google/go-github/...

tests/integration/authorizations_test.go

@@ -3,6 +3,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build integration
+
 package tests
 
 import (

tests/integration/doc.go

@@ -0,0 +1,6 @@
+// Package tests contains integration tests.
+//
+// These tests call the live GitHub API, and therefore require a little more
+// setup to run.  See https://github.com/google/go-github/tree/master/tests/integration
+// for more information.
+package tests

tests/integration/github_test.go

@@ -3,9 +3,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// These tests call the live GitHub API, and therefore require a little more
-// setup to run.  See https://github.com/google/go-github/tree/master/tests/integration
-// for more information
+// +build integration
 
 package tests