Update docs/comments to use setup/teardown blocks vs. methods.

As discussed in the minitest-spec-rails issue (http://git.io/vlHxx) Rails uses setup/teardown callbacks. Defining `setup` or `teardown` methods vs. blocks will yield inconsistent behavior in the callback chain.

activesupport/lib/active_support/log_subscriber/test_helper.rb

@@ -10,7 +10,7 @@ class LogSubscriber
     #   class SyncLogSubscriberTest < ActiveSupport::TestCase
     #     include ActiveSupport::LogSubscriber::TestHelper
     #
-    #     def setup
+    #     setup do
     #       super
     #       ActiveRecord::LogSubscriber.attach_to(:active_record)
     #     end

guides/source/active_model_basics.md

@@ -428,7 +428,7 @@ the Active Model API.
     class PersonTest < ActiveSupport::TestCase
       include ActiveModel::Lint::Tests
 
-      def setup
+      setup do
         @model = Person.new
       end
     end

guides/source/testing.md

@@ -908,12 +908,12 @@ require 'test_helper'
 
 class ArticlesControllerTest < ActionController::TestCase
   # called before every single test
-  def setup
+  setup do
     @article = articles(:one)
   end
 
   # called after every single test
-  def teardown
+  teardown do
     # when controller is using cache it may be a good idea to reset it afterwards
     Rails.cache.clear
   end