Harden new parallel string_agg/array_agg regression test

Per buildfarm member mandrill, it seems that
max_parallel_workers_per_gather may not always be set to the default value
of 2 when the new test added in 16fd03e is executed.  Here let's just
explicitly set that to 2 so that the planner never opts to use more than
that many parallel workers.

Author: david-rowley

src/test/regress/expected/aggregates.out

@@ -1872,6 +1872,7 @@ set parallel_tuple_cost TO 0;
 set parallel_leader_participation TO 0;
 set min_parallel_table_scan_size = 0;
 set bytea_output = 'escape';
+set max_parallel_workers_per_gather = 2;
 -- create a view as we otherwise have to repeat this query a few times.
 create view v_pagg_test AS
 select

src/test/regress/sql/aggregates.sql

@@ -728,6 +728,7 @@ set parallel_tuple_cost TO 0;
 set parallel_leader_participation TO 0;
 set min_parallel_table_scan_size = 0;
 set bytea_output = 'escape';
+set max_parallel_workers_per_gather = 2;
 
 -- create a view as we otherwise have to repeat this query a few times.
 create view v_pagg_test AS