fw4: fix handling the ipset "comment" option

fw4: fix handling the ipset "comment" option

The comment option for ipset definitions is incorrectly declared as bool
and not actually used anywhere in the nftables output rendering.

Solve this issue by changing it to the proper "string" type and expose
the user configured comment as "comment" property in the generated nftables
output.

Also add some initial test coverage for ipset declarations to better spot
such inconsistencies in the future.

Ref: https://github.com/openwrt/luci/pull/6187#issuecomment-1374506633
Reported-by: Paul Dee <itsascambutmailmeanyway@gmail.com>
Signed-off-by: Jo-Philipp Wich <jo@mein.io>