According to the readme for the %INPUT parameter, it should be possible:
Test input, in the form of an HTML template or fragment. A resource name can also be specified between parenthesis, like %INPUT (file:/home/user/myproject/src/main/resources/templates/mytemplate.html). This parameter is required.
Although I'm not sure what the output to match on would be. I expect it would just be the fragment execution as well.
Ow nice! A DOM selector sounds really need! Is the TestFragmentTestFieldEvaluator somewere in the Thymeleaf testing dependency? I couldn't find it and %FRAGMENT[evaluator] org.thymeleaf.engine21.domselector.TestFragmentTestFieldEvaluator throws a ClassNotFoundException.