json: icmp: move expected parts to json.output

json: icmp: move expected parts to json.output

Phil Sutter says:
In general, *.t.json files should contain JSON equivalents for rules as
they are *input* into nft. So we want them to be as close to the
introductory standard syntax comment as possible.

Undo earlier change and place the expected dependency added by
nft internals to json.output rather than icmp.t.json.

Signed-off-by: Florian Westphal <fw@strlen.de>