tests/shell: test for maximum length of "comment" in "comments_objects_0"

tests/shell: test for maximum length of "comment" in "comments_objects_0"

The comment length is limited to NFTNL_UDATA_COMMENT_MAXLEN. Test for
that.

Adjust an existing test for that.

Also rename $EXPECTED to $RULESET. We don't compare the value of
$EXPECTED against the actually configured rules. It also wouldn't work,
because the input is not normalized and wouldn't match. It also isn't
necessary, because there is a .nft dump file.

Signed-off-by: Thomas Haller <thaller@redhat.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>