pf tests: check cleared time when zeroing stats for table addresses

Verify that we reset the cleared time when we zero an address' counters in
a table.

PR: 282877, 282984
Reviewed by: kp
MFC after: 2 weeks
Signed-off-by: Leonid Evdokimov <leon@darkk.net.ru>
Differential Revision: https://reviews.freebsd.org/D48242

(cherry picked from commit 0749d8134300b8e3c956e161890ab496247d2542)