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.

1 files changed, 2 insertions, 2 deletions

diff --git a/actionmailer/test/delivery_methods_test.rb b/actionmailer/test/delivery_methods_test.rb
index d17e774092..2786fe0d07 100644
--- a/actionmailer/test/delivery_methods_test.rb
+++ b/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