proxy/iptables: Improve the sorting logic in TestOverallIPTablesRules…

…WithMultipleServices

The test was sorting the iptables output so as to not depend on the
order that services get processed in, but this meant it wasn't
checking the relative ordering of rules (and in fact, the ordering of
the rules in the "expected" string was wrong, in a way that would
break things if the rules had actually been generated in that order).

Add a more complicated sorting function that sorts services
alphabetically while preserving the ordering of rules within each
service.