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.