Set client encoding to UTF8 for the citext regression script

Problem introduced with non-ascii characters in commit f246499764
and discovered on various buildfarm animals.

contrib/citext/expected/citext.out

@@ -1,6 +1,8 @@
 --
 --  Test citext datatype
 --
+--- script setup
+set client_encoding = 'utf8';
 CREATE EXTENSION citext;
 -- Check whether any of our opclasses fail amvalidate
 SELECT amname, opcname

contrib/citext/expected/citext_1.out

@@ -1,6 +1,8 @@
 --
 --  Test citext datatype
 --
+--- script setup
+set client_encoding = 'utf8';
 CREATE EXTENSION citext;
 -- Check whether any of our opclasses fail amvalidate
 SELECT amname, opcname

contrib/citext/sql/citext.sql

@@ -2,6 +2,9 @@
 --  Test citext datatype
 --
 
+--- script setup
+set client_encoding = 'utf8';
+
 CREATE EXTENSION citext;
 
 -- Check whether any of our opclasses fail amvalidate