Tiny documentation fixes [ci skip]

* Add missing `def` and remove useless `do` keywords.
* Move `:nodoc:` in front of the methods' definition so that methods
  under these ones are correctly visible on the API.

guides/source/testing.md

@@ -530,7 +530,7 @@ Let me take you through one such test, `test_should_get_index` from the file `ar
 ```ruby
 # articles_controller_test.rb
 class ArticlesControllerTest < ActionController::TestCase
-  test_should_get_index do
+  def test_should_get_index
     get :index
     assert_response :success
     assert_includes @response.body, 'Articles'
@@ -572,7 +572,7 @@ NOTE: If you try running `test_should_create_article` test from `articles_contro
 Let us modify `test_should_create_article` test in `articles_controller_test.rb` so that all our test pass:
 
 ```ruby
-test_should_create_article do
+def test_should_create_article
   assert_difference('Article.count') do
     post :create, params: { article: { title: 'Some title' } }
   end

railties/lib/rails/paths.rb

@@ -123,8 +123,7 @@ module Rails
         options[:load_path]     ? load_path!     : skip_load_path!
       end
 
-      # :nodoc:
-      def absolute_current
+      def absolute_current # :nodoc:
         File.expand_path(@current, @root.path)
       end
 
@@ -180,8 +179,7 @@ module Rails
         @paths
       end
 
-      # :nodoc:
-      def extensions
+      def extensions # :nodoc:
         $1.split(',') if @glob =~ /\{([\S]+)\}/
       end