As long as you only have a single test, then the minimal code needed to pass the test is truly return 120;, and you can easily keep it for that as long as you don't have any more tests.
This allows you to postpone further design until you actually write the tests that exercise OTHER return values of this method.
Please remember that the test is the runnable version of your specification, and if all thethat specification says is that f(6)=120 then that fits the bill perfectly.