Quite right the FI test will not qualify you for renewal of an SEP rating. It is a Test, not an instructional flight, and is not for an aircraft or IMC rating, so does not count as your Dual Flight from which FIs are not exempt.
Whilst many FIEs will require you to have a valid SEP rating when presenting yourself for a FI revalidation test, they may if asked nicely, be prepared to conduct both tests on the same flight, for which they can quite rightly charge two test fees because they have two jobs to do.