anchor.diff:  Each test case follows the same structure:
   
   [source]
  +----
   test 'description of test' do
     # test logic
   end
  +----
   
   At the moment, the tests are quite primitive.
   Here's how a typical test operates:
  
positive.code: = Contributing
  // [settings]
  :idprefix:
  :idseparator: -
  :source-language: ruby
  :language: {source-language}
  // [URIs]
  :repo-uri: https://github.com/asciidoctor/asciidoctor
  :base-help-uri: https://help.github.com/articles
  :issues-uri: {repo-uri}/issues
  
negative.code:   assert_css '.openblock p', result, 1
    assert_xpath '/div[@class="openblock"]//p[text()="This is an open block."]', result, 1
  end
  ----
  
  As you can see, several helpers are used to facilitate the test scenario.
  The `render_embedded_string` invokes Asciidoctor's render method with the header and footer option disabled.
  This method is ideal for unit-level tests.
  If you need to test the whole document, use `render_string` instead.
  The `assert_css` and `assert_xpath` assertion methods take a CSS or XPath selector, respectively, the rendered result and the number of expected matches.
  You can also use built-in assertions in Ruby's test library.
  
  To run all the tests, simply execute `rake`:
  
   $ rake
  
  NOTE: The tests should only take a few seconds to run using Ruby 2.1.
  
  If you want to run a single test file, you can use `ruby`:
  
   $ ruby test/blocks_test.rb
  
  