pf tests: fix intermittent mld test failures

We can't reliably check for the absence of replies to our MLD queries (because
a host may announce its multicast subscriptions), so enable pf logging and check
for the relevant error message instead.

PR: 289821
MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D52762