Is it possible to extend twig with 'Twig_SimpleTest'? I was on the Twig StackExchange forum and found an answer that suited my needs.

But I can't for the life of me figure out how to implement this into a Craft CMS Plugin.

I'm familiar with Twig_Function_Method and Twig_Filter_Method, but I can't find anything on Twig_SimpleTest. 

Has the option to include "tests" been omitted from Craft CMS?
When you call Craft's TemplatesService->getTwig() (craft()-&gt;templates-&gt;getTwig()), you get an instance of the Twig environment and from there you could be able to call addTest.

100% untested, but something like this should work:

$twig = craft()-&gt;templates-&gt;getTwig();
$twig-&gt;addTest(new My_Twig_SimpleTest());