Correctly skip rtsock_l3 test when if_epair is not available

This logic was accidentally changed in 67f2f67fc8ce8484066e95dabac5af892888fca1

Reported By: olivier
X-MFC-With: 67f2f67fc8ce8484066e95dabac5af892888fca1
Tested By: kp
Reviewed By: kp, melifaro
Fixes: 67f2f67fc8ce ("Update rtsock_l3 test after 2fe5a79425c7")
Differential Revision: https://reviews.freebsd.org/D29944