Tests: Only create publicuser if necessary

Also, don't try to drop it since it's used by default
CARTO installation

test/CDB_UserTablesTest.sql

@@ -1,4 +1,18 @@
-CREATE ROLE publicuser LOGIN;
+SET client_min_messages TO ERROR;
+DO
+$do$
+BEGIN
+   IF NOT EXISTS (
+      SELECT *
+      FROM   pg_catalog.pg_user
+      WHERE  usename = 'publicuser') THEN
+
+      CREATE ROLE publicuser LOGIN;
+   END IF;
+END
+$do$;
+SET client_min_messages TO NOTICE;
+
 CREATE TABLE pub(a int);
 CREATE TABLE prv(a int);
 GRANT SELECT ON TABLE pub TO publicuser;
@@ -16,4 +30,3 @@ SELECT 'private_publicuser',CDB_UserTables('private') ORDER BY 2;
 \c contrib_regression postgres
 DROP TABLE pub;
 DROP TABLE prv;
-DROP ROLE publicuser;