Previously, only the PrismRenderParser or RipperRenderParser would be
tested depending on if the Prism gem is available. This meant that
PrismRenderParser was being tested on Ruby 3.3 and RipperRenderParser
was tested on Ruby < 3.3. Additionally, if someone were to add prism to
the rails/rails Gemfile because they wrote a tool that uses it then the
RipperRenderParser would end up completely untested.

This commit is a small refactor to enable testing both RenderParsers in
all Ruby versions so that the prism gem can be added to the Gemfile.