This will in particular add some good test coverage for
inet_cidr_ntop.c, which was previously completely uncovered.
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://www.postgresql.org/message-id/flat/cb0c4662-4596-dab4-7f64-839c5e8582c8%40enterprisedb.com