Testing AbstractProcessor (Java's annotation handling at compile time)

I am writing a library which relays on the AbstractProcessor class, since I want to write a great library I want to have a good coverage too. Since the preprocessor works at the compile time I am not sure how to test that code.

I have some test scenarios in mind where the build should fail. However how can I test that? Do I have to execute gradle and check the exit code? Is there a clean way to verify that the build fails caused by an expected cause or do I need to write also some kind of parser? That would be IMHO a huge overhead just for a good coverage.