MFC r361065: pf tests: fix up a couple WARNS= 6 nits

MFC r361065: pf tests: fix up a couple WARNS= 6 nits

common_init_tbl is only used within this single CU, so it should be marked
static.

WARNS=6 also complained about the var defined by
`ATF_TC_WITH_CLEANUP(getastats);` being unused, which turns out to be
because it's not been hooked up in ATF_TP_ADD_TCS. kp@ did not immediately
recall any reason for this, and the case passes on my local system, so hook
it up.

Note that I've not yet set WARNS= 6 here. Investigation is underway to see
if we can feasibly default WARNS to 6 for src builds to catch directories
too deep to inherit a WARNS from the top-level subdirectories' Makefile.inc.
Those particular WARNS settings will be subsequently removed as they become
redundant with a more-global default.