Writing extensions for CheckStyle, e.g. custom checks, is easy, regression testing them is not. This framework makes it easy to use JUNIT for exactly that purpose.
There is no downloadable JAR file (yet), but the source code is available via SVN at https://svn.code.sf.net/p/junit-cs/code.
An example test case built atop junit-cs can be found here.
Contributions of any kind welcome!