Define a Redcarpet::TestCase class

Let's have an entry point for our test suite to add eventual helpers or
define custom stuff instead of relying on Test::Unit::TestCase directly.
Avoid defining the html_equal helper at the top level

Since we now have a Redcarpet::TestCase class, let's push any helper
here and avoid defining top level methods.
Rely on #scan instead of #split

Let's rely on #scan in combination with #length instead of #split to
make the assertion more clear. Previously, we were asserting that the
array returned by #split have a length of n+1 where n is the actual
number of elements. With #scan, this is now a lot more easy to read.
Merge pull request #381 from vmg/test-suite

Tiny improvements to the test suite