optimize: select merge criteria based on candidates rules

optimize: select merge criteria based on candidates rules

Select the merge criteria based on the statements that are used
in the candidate rules, instead of using the list of statements
in the given chain.

Update tests to include a rule with a verdict, which triggers
the bug described in the bugzilla ticket.

Closes: https://bugzilla.netfilter.org/show_bug.cgi?id=1657
Fixes: 0a6dbfce6dc3 ("optimize: merge nat rules with same selectors into map")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>