psql: Make output of \dD more stable

\dD showed domain check constraints in arbitrary order, which can
cause regression test failures, which was exposed by commit
9895b35cb8.  To fix, order the constraints by conname, which matches
what psql does in other queries listing constraints.

src/bin/psql/describe.c

@@ -4449,7 +4449,7 @@ listDomains(const char *pattern, bool verbose, bool showSystem)
 					  "       CASE WHEN t.typnotnull THEN 'not null' END as \"%s\",\n"
 					  "       t.typdefault as \"%s\",\n"
 					  "       pg_catalog.array_to_string(ARRAY(\n"
-					  "         SELECT pg_catalog.pg_get_constraintdef(r.oid, true) FROM pg_catalog.pg_constraint r WHERE t.oid = r.contypid AND r.contype = 'c'\n"
+					  "         SELECT pg_catalog.pg_get_constraintdef(r.oid, true) FROM pg_catalog.pg_constraint r WHERE t.oid = r.contypid AND r.contype = 'c' ORDER BY r.conname\n"
 					  "       ), ' ') as \"%s\"",
 					  gettext_noop("Schema"),
 					  gettext_noop("Name"),

src/test/regress/expected/domain.out

@@ -789,7 +789,7 @@ alter domain con add check (VALUE > 0);
                                          List of domains
  Schema | Name |  Type   | Collation | Nullable | Default |                Check                 
 --------+------+---------+-----------+----------+---------+--------------------------------------
- public | con  | integer |           |          |         | CHECK (VALUE < 34) CHECK (VALUE > 0)
+ public | con  | integer |           |          |         | CHECK (VALUE > 0) CHECK (VALUE < 34)
 (1 row)
 
 insert into domcontest values (-5); -- fails