Using "src/test/java" is not a bad idea. I can't imagine a scenario where you'd have to package them as a jar. If you going to run them on some kind of CI server then you'd probably use maven anyway. Also why would you ever need to write unit tests for selenium tests?
The way I've done it in the past is using something like "src/acceptance-test/java", but that requires some adjustments in your build script and ide.