netpfil tests: disable ICMPv6 rate limiting in the test jail

The dummynet test uses flood ping as source of traffic, so the rate
limiting of ICMP replies broke the test.

Fixes: 32aeee8ce7e72738fff236ccd5629d55035458f8
(cherry picked from commit 75d15e893b14188b83c5fb5e4979fa21c557934f)