Clean up a bit the Markdown tests
Clean up a bit the Markdown tests

Use the available test helpers like `strip_heredoc` and rely on the
`render` helper rather than a mix of `render_with` and `@markdown.render`.

fukayatsu/redcarpet,vmg/redcarpet,increments/greenmat,increments/greenmat,fukayatsu/redcarpet,increments/greenmat,vmg/redcarpet,kaneshin/redcarpet,kaneshin/redcarpet,increments/greenmat,vmg/redcarpet,vmg/redcarpet,kaneshin/redcarpet,fukayatsu/redcarpet,kaneshin/redcarpet,fukayatsu/redcarpet