Add test case for abbrev(cidr)

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

src/test/regress/expected/inet.out

@@ -88,6 +88,28 @@ SELECT i AS inet, host(i), text(i), family(i) FROM INET_TBL;
  ::4.3.2.1/24     | ::4.3.2.1     | ::4.3.2.1/24     |      6
 (17 rows)
 
+SELECT c AS cidr, abbrev(c) FROM INET_TBL;
+        cidr        |       abbrev       
+--------------------+--------------------
+ 192.168.1.0/24     | 192.168.1/24
+ 192.168.1.0/26     | 192.168.1.0/26
+ 192.168.1.0/24     | 192.168.1/24
+ 192.168.1.0/24     | 192.168.1/24
+ 192.168.1.0/24     | 192.168.1/24
+ 192.168.1.0/24     | 192.168.1/24
+ 10.0.0.0/8         | 10/8
+ 10.0.0.0/32        | 10.0.0.0/32
+ 10.1.2.3/32        | 10.1.2.3/32
+ 10.1.2.0/24        | 10.1.2/24
+ 10.1.0.0/16        | 10.1/16
+ 10.0.0.0/8         | 10/8
+ 10.0.0.0/8         | 10/8
+ 10.0.0.0/8         | 10/8
+ 10:23::f1/128      | 10:23::f1/128
+ 10:23::8000/113    | 10:23::8000/113
+ ::ffff:1.2.3.4/128 | ::ffff:1.2.3.4/128
+(17 rows)
+
 SELECT c AS cidr, broadcast(c),
   i AS inet, broadcast(i) FROM INET_TBL;
         cidr        |    broadcast     |       inet       |               broadcast               

src/test/regress/sql/inet.sql

@@ -34,6 +34,7 @@ SELECT c AS cidr, i AS inet FROM INET_TBL;
 -- now test some support functions
 
 SELECT i AS inet, host(i), text(i), family(i) FROM INET_TBL;
+SELECT c AS cidr, abbrev(c) FROM INET_TBL;
 SELECT c AS cidr, broadcast(c),
   i AS inet, broadcast(i) FROM INET_TBL;
 SELECT c AS cidr, network(c) AS "network(cidr)",