Just simply ask for that, and they usually know what to do.
They should write a test and stash the fix they just wrote. In this state it should fail. Once they re-apply the patch, it should become green and this gave use enough information to deduce that the test does test something.
They should write a test and stash the fix they just wrote. In this state it should fail. Once they re-apply the patch, it should become green and this gave use enough information to deduce that the test does test something.