Harden new test case against force_parallel_mode = regress.

Per buildfarm: worker processes can't see a role created in
the current transaction.

src/test/regress/expected/psql.out

@@ -5248,9 +5248,9 @@ reset work_mem;
 (1 row)
 
 -- check \df+
-begin;
 -- we have to use functions with a predictable owner name, so make a role
 create role regress_psql_user superuser;
+begin;
 set session authorization regress_psql_user;
 create function psql_df_internal (float8)
   returns float8
@@ -5275,6 +5275,7 @@ comment on function psql_df_plpgsql () is 'some comment';
 (3 rows)
 
 rollback;
+drop role regress_psql_user;
 -- check \sf
 \sf information_schema._pg_expandarray
 CREATE OR REPLACE FUNCTION information_schema._pg_expandarray(anyarray, OUT x anyelement, OUT n integer)

src/test/regress/sql/psql.sql

@@ -1276,9 +1276,9 @@ reset work_mem;
 \do && anyarray *
 
 -- check \df+
-begin;
 -- we have to use functions with a predictable owner name, so make a role
 create role regress_psql_user superuser;
+begin;
 set session authorization regress_psql_user;
 
 create function psql_df_internal (float8)
@@ -1297,6 +1297,7 @@ comment on function psql_df_plpgsql () is 'some comment';
 
 \df+ psql_df_*
 rollback;
+drop role regress_psql_user;
 
 -- check \sf
 \sf information_schema._pg_expandarray