Remove unportable use of timezone in recent test

Per buildfarm member snapper

Discussion: https://postgr.es/m/129951.1659812518@sss.pgh.pa.us

Author: alvherre

src/test/modules/brin/t/02_wal_consistency.pl

@@ -44,7 +44,7 @@
 do
 $$
 declare
-  current timestamp with time zone := '2019-03-27 08:14:01.123456789 America/Punta_Arenas';
+  current timestamp with time zone := '2019-03-27 08:14:01.123456789 UTC';
 begin
   loop
     insert into tbl_timestamp0 select i from