selftests: netfilter: br_netfilter.sh: move to lib.sh infra

Also, fix two issues reported by Pablo Neira:
1. Must modprobe br_netfilter in case its not loaded,
   else sysctl cannot be set.
2. ping for netns4 fails if rp_filter is enabled in bridge netns,
   so set all and default to 0.

Signed-off-by: Florian Westphal <fw@strlen.de>
Link: https://lore.kernel.org/r/20240411233624.8129-4-fw@strlen.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>