Add non-existent DIMMs test case in spd_cache-test. BUG=b:213964936 TEST=make unit-tests PASSED Signed-off-by: Eric Lai <eric_lai@quanta.corp-partner.google.com> Change-Id: I3c8aa92ee0cfd5908399f4bbd305f8f306571d40 Reviewed-on: https://review.coreboot.org/c/coreboot/+/63643 Tested-by: build bot (Jenkins) <no-reply@coreboot.org> Reviewed-by: Paul Menzel <paulepanter@mailbox.org> Reviewed-by: Tim Wawrzynczak <inforichland@gmail.com> Reviewed-by: Jakub Czapiga <jacz@semihalf.com>