The helpers were tested in isolation but nothing tied them together, and
what actually has to hold is that the key SetCIDR writes for a decision is
one of the keys GetCIDR looks up for an IP that decision covers. A
formatting change on either side would have silently stopped every range
decision from matching with all tests green.
TestCIDRKeys_MatchNormalizeCIDR pins that both ways, including the cases
worth being explicit about: a decision that is not on a network address,
IPv4 mapped clients against an IPv4 range, and that the two families do not
mix. Dropping the mask in cidrKey fails 9 of its cases.
Also covers CIDRLookupKeys against CIDRKeys length by length, the IPv6 side
of the network address test, and CIDRPrefixLen.