pf tests: More set skip on <ifgroup> tests

Test the specific case reported in PR 255852. Clearing the skip flag
on groups was broken because pfctl couldn't work out if a kif was a
group or not, because the kernel no longer set the pfik_group pointer.

PR: 255852
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D30285