use :test delivery method for `base_test.rb`

This solves errors like:

```
BaseTest#test_you_can_register_multiple_observers_to_the_mail_object_that_both_get_informed_on_email_delivery:
Errno::ECONNREFUSED: Connection refused - connect(2) for "localhost" port 25
    /Users/senny/.rbenv/versions/2.1.2/lib/ruby/2.1.0/net/smtp.rb:541:in `initialize'
    /Users/senny/.rbenv/versions/2.1.2/lib/ruby/2.1.0/net/smtp.rb:541:in `open'
    /Users/senny/.rbenv/versions/2.1.2/lib/ruby/2.1.0/net/smtp.rb:541:in `tcp_socket'
    /Users/senny/.rbenv/versions/2.1.2/lib/ruby/2.1.0/net/smtp.rb:551:in `block in do_start'
    /Users/senny/.rbenv/versions/2.1.2/lib/ruby/2.1.0/timeout.rb:91:in `block in timeout'
    /Users/senny/.rbenv/versions/2.1.2/lib/ruby/2.1.0/timeout.rb:101:in `call'
    /Users/senny/.rbenv/versions/2.1.2/lib/ruby/2.1.0/timeout.rb:101:in `timeout'
    /Users/senny/.rbenv/versions/2.1.2/lib/ruby/2.1.0/net/smtp.rb:550:in `do_start'
    /Users/senny/.rbenv/versions/2.1.2/lib/ruby/2.1.0/net/smtp.rb:520:in `start'
    /Users/senny/Projects/rails/.bundle/gems/mail-2.6.1/lib/mail/network/delivery_methods/smtp.rb:112:in `deliver!'
    /Users/senny/Projects/rails/.bundle/gems/mail-2.6.1/lib/mail/message.rb:2136:in `do_delivery'
    /Users/senny/Projects/rails/.bundle/gems/mail-2.6.1/lib/mail/message.rb:232:in `block in deliver'
    /Users/senny/Projects/rails/actionmailer/lib/action_mailer/base.rb:528:in `block in deliver_mail'
    /Users/senny/Projects/rails/activesupport/lib/active_support/notifications.rb:164:in `block in instrument'
    /Users/senny/Projects/rails/activesupport/lib/active_support/notifications/instrumenter.rb:20:in `instrument'
    /Users/senny/Projects/rails/activesupport/lib/active_support/notifications.rb:164:in `instrument'
    /Users/senny/Projects/rails/actionmailer/lib/action_mailer/base.rb:526:in `deliver_mail'
    /Users/senny/Projects/rails/.bundle/gems/mail-2.6.1/lib/mail/message.rb:232:in `deliver'
    /Users/senny/Projects/rails/actionmailer/test/base_test.rb:598:in `block (2 levels) in <class:BaseTest>'
    /Users/senny/Projects/rails/actionmailer/test/base_test.rb:801:in `mail_side_effects'
    /Users/senny/Projects/rails/actionmailer/test/base_test.rb:593:in `block in <class:BaseTest>'
```
where the `:smtp` delivery method was leaked over to other test cases.