pf tests: make mbuf:inet6_in_mbuf_len more robust

The mbuf:inet6_in_mbuf_len test sometimes fails because it encountered
unexpected extra packets. These turn out to be MLD packets, so block these
packets on the host with pf so they don't disturb what we're actually trying
to test.

Reviewed by: igoro
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D51408