pf: assert dir state on pf_test{,6}

The intent is to line up various enums so that branching in the lines of:

idx = (dir == PF_IN ? PF_SK_WIRE : PF_SK_STACK);

is avoided.

Reviewed by: kp
Sponsored by: Rubicon Communications, LLC ("Netgate")

(cherry picked from commit 3e875f952af05697807139b460e5124eb5f6137b)