DRYing up controller tests and mailer tests using superclasses

Hi!

Currently both, the controller/functional tests and mailer tests,
contain a lot of cruft. I've created two superclasses to DRY them up a
bit. Here are some examples how tests using those superclasses will
look:

http://pastie.caboo.se/66139

Ticket with the patch:

http://dev.rubyonrails.org/ticket/8521

Known issues:

* Since we don't explicitly setup the @request and @response objects
anymore, we should probably show how to use them in the dummy test_foo
method. Could be something like:

def test_foo
  @request.host = "foo.bar.com"
  assert true
  ...
end

* The creation of the method_added-method in ControllerTest and
MailerTest should probably be DRYed up and put into RailsTest.

In any case, feedback on this is appreciated.

Thanks!

Ciao!
Florian