Enforce non-parallel plan when calling current_schema() in newly-added test

current_schema() gets called in the recently-added regression test from
c5660e0, and can be used in a parallel context, causing its call to fail
when creating a temporary schema.

Per buildfarm members crake and lapwing.

Discussion: https://postgr.es/m/20190118005949.GD1883@paquier.xyz

src/test/regress/expected/temp.out

@@ -360,8 +360,11 @@ prepare transaction 'twophase_tab';
 ERROR:  cannot PREPARE a transaction that has operated on temporary tables
 -- Corner case: current_schema may create a temporary schema if namespace
 -- creation is pending, so check after that.  First reset the connection
--- to remove the temporary namespace.
+-- to remove the temporary namespace, and make sure that non-parallel plans
+-- are used.
 \c -
+SET max_parallel_workers = 0;
+SET max_parallel_workers_per_gather = 0;
 SET search_path TO 'pg_temp';
 BEGIN;
 SELECT current_schema() ~ 'pg_temp' AS is_temp_schema;

src/test/regress/sql/temp.sql

@@ -274,8 +274,11 @@ prepare transaction 'twophase_tab';
 
 -- Corner case: current_schema may create a temporary schema if namespace
 -- creation is pending, so check after that.  First reset the connection
--- to remove the temporary namespace.
+-- to remove the temporary namespace, and make sure that non-parallel plans
+-- are used.
 \c -
+SET max_parallel_workers = 0;
+SET max_parallel_workers_per_gather = 0;
 SET search_path TO 'pg_temp';
 BEGIN;
 SELECT current_schema() ~ 'pg_temp' AS is_temp_schema;