Move assert_renders to Redcarpet::TestCase

This helper may be useful in other test files so let's move it to our
own test case class.

Also define a method in order to avoid allocation of a new object
anytime we call the helper.
Switch the arguments order of assert_renders

For better failure messages, let's switch the arguments order as we are
implicitly relying on assert_equal which expect to have the expected
object first, and the given one then.