Fix failures in incremental_sort due to number of workers

The last test in incremental_sort suite prints a parallel plan, but some
of the buildfarm animals have custom max_parallel_workers_per_gather
values, causing failures. Fixed by setting the GUC to an explicit value.

Discussion: https://postgr.es/m/CAPpHfds1waRZ=NOmueYq0sx1ZSCnt+5QJvizT8ndT2=etZEeAQ@mail.gmail.com

src/test/regress/expected/incremental_sort.out

@@ -1402,6 +1402,7 @@ set min_parallel_table_scan_size = '1kB';
 set min_parallel_index_scan_size = '1kB';
 set parallel_setup_cost = 0;
 set parallel_tuple_cost = 0;
+set max_parallel_workers_per_gather = 2;
 create table t (a int, b int, c int);
 insert into t select mod(i,10),mod(i,10),i from generate_series(1,10000) s(i);
 create index on t (a);

src/test/regress/sql/incremental_sort.sql

@@ -198,6 +198,7 @@ set min_parallel_table_scan_size = '1kB';
 set min_parallel_index_scan_size = '1kB';
 set parallel_setup_cost = 0;
 set parallel_tuple_cost = 0;
+set max_parallel_workers_per_gather = 2;
 
 create table t (a int, b int, c int);
 insert into t select mod(i,10),mod(i,10),i from generate_series(1,10000) s(i);