tests: os: expand iptables rules tests to include ip6tables rules

[rcooke-warwick]

The supervisor uses different iptables binaries for ipv4 and ipv6 rules, so we should check it adds rules for both, inc ase there is a problem with ip6tables

Change-type: patch

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[kb2ma]

Looks good to me, @rcooke-warwick. I verified that the BALENA-FIREWALL chain has at least 10 entries on a recent device. Is it feasible to run at least this one test locally to confirm?

[pipex]

Looks good to me too @rcooke-warwick

[rcooke-warwick]

depends on https://github.com/balena-os/meta-balena/pull/3484 to be able to pass - at the moment it correctly fails