tests, define `Rails.root` before loading Action Mailer.

Since `bin/test` would define `Rails.root` before loading AM but `bundle
exec rake` would define it after loading AM, this lead to the following
test failure when using `bin/test`:

```
--- expected
+++ actual
@@ -1 +1 @@
-{:location=>"/var/folders/_r/9kh50y4j0vn7zgg2p65z1lcm0000gn/T/mails"}
+{:location=>"/Users/senny/Projects/rails/actionmailer/tmp/mails"}
```

This patch ensures that `Rails.root` is defined when loading AM to
achieve consistent test results.

actionmailer/test/abstract_unit.rb

@@ -8,6 +8,12 @@ silence_warnings do
   Encoding.default_external = "UTF-8"
 end
 
+module Rails
+  def self.root
+    File.expand_path('../', File.dirname(__FILE__))
+  end
+end
+
 require 'active_support/testing/autorun'
 require 'active_support/testing/method_call_assertions'
 require 'action_mailer'
@@ -26,12 +32,6 @@ I18n.enforce_available_locales = false
 FIXTURE_LOAD_PATH = File.expand_path('fixtures', File.dirname(__FILE__))
 ActionMailer::Base.view_paths = FIXTURE_LOAD_PATH
 
-module Rails
-  def self.root
-    File.expand_path('../', File.dirname(__FILE__))
-  end
-end
-
 # Skips the current run on Rubinius using Minitest::Assertions#skip
 def rubinius_skip(message = '')
   skip message if RUBY_ENGINE == 'rbx'

actionmailer/test/delivery_methods_test.rb

@@ -31,8 +31,8 @@ class DefaultsDeliveryMethodsTest < ActiveSupport::TestCase
     assert_equal settings, ActionMailer::Base.smtp_settings
   end
 
-  test "default file delivery settings" do
-    settings = {location: "#{Dir.tmpdir}/mails"}
+  test "default file delivery settings (with Rails.root)" do
+    settings = {location: "#{Rails.root}/tmp/mails"}
     assert_equal settings, ActionMailer::Base.file_settings
   end