Make new regression indpendent of max_parallel_workers_per_gather.

The tests in e2f1eb0e ("Implement partition-wise
grouping/aggregation.") weren't independent of the server's
max_parallel_workers_per_gather setting.  I (Andres) find it useful to
locally run with that disabled, and the aforementioned patch broke
this.

Author: Jeevan Chalke
Discussion:
    https://postgr.es/m/20180322210703.qmga3vsxqmiiypci@alap3.anarazel.de
    https://postgr.es/m/CAM2+6=UNWGKTgh9aOn4=SQ72HfFzbVFseh9=5N54bD6KB+D9OQ@mail.gmail.com

src/test/regress/expected/partition_aggregate.out

@@ -6,6 +6,8 @@
 SET enable_partitionwise_aggregate TO true;
 -- Enable partitionwise join, which by default is disabled.
 SET enable_partitionwise_join TO true;
+-- Disable parallel plans.
+SET max_parallel_workers_per_gather TO 0;
 --
 -- Tests for list partitioned tables.
 --
@@ -921,6 +923,8 @@ ALTER TABLE pagg_tab_ml_p3 ATTACH PARTITION pagg_tab_ml_p3_s1 FOR VALUES FROM (0
 ALTER TABLE pagg_tab_ml ATTACH PARTITION pagg_tab_ml_p3 FOR VALUES FROM (20) TO (30);
 INSERT INTO pagg_tab_ml SELECT i % 30, i % 10, to_char(i % 4, 'FM0000') FROM generate_series(0, 29999) i;
 ANALYZE pagg_tab_ml;
+-- For Parallel Append
+SET max_parallel_workers_per_gather TO 2;
 -- Full aggregation at level 1 as GROUP BY clause matches with PARTITION KEY
 -- for level 1 only. For subpartitions, GROUP BY clause does not match with
 -- PARTITION KEY, but still we do not see a partial aggregation as array_agg()
@@ -1146,7 +1150,6 @@ SELECT a, sum(b), count(*) FROM pagg_tab_ml GROUP BY a, b, c HAVING avg(b) > 7 O
 (12 rows)
 
 -- Parallelism within partitionwise aggregates
-SET max_parallel_workers_per_gather TO 2;
 SET min_parallel_table_scan_size TO '8kB';
 SET parallel_setup_cost TO 0;
 -- Full aggregation at level 1 as GROUP BY clause matches with PARTITION KEY

src/test/regress/sql/partition_aggregate.sql

@@ -7,6 +7,8 @@
 SET enable_partitionwise_aggregate TO true;
 -- Enable partitionwise join, which by default is disabled.
 SET enable_partitionwise_join TO true;
+-- Disable parallel plans.
+SET max_parallel_workers_per_gather TO 0;
 
 --
 -- Tests for list partitioned tables.
@@ -206,6 +208,9 @@ ALTER TABLE pagg_tab_ml ATTACH PARTITION pagg_tab_ml_p3 FOR VALUES FROM (20) TO
 INSERT INTO pagg_tab_ml SELECT i % 30, i % 10, to_char(i % 4, 'FM0000') FROM generate_series(0, 29999) i;
 ANALYZE pagg_tab_ml;
 
+-- For Parallel Append
+SET max_parallel_workers_per_gather TO 2;
+
 -- Full aggregation at level 1 as GROUP BY clause matches with PARTITION KEY
 -- for level 1 only. For subpartitions, GROUP BY clause does not match with
 -- PARTITION KEY, but still we do not see a partial aggregation as array_agg()
@@ -238,7 +243,6 @@ SELECT a, sum(b), count(*) FROM pagg_tab_ml GROUP BY a, b, c HAVING avg(b) > 7 O
 
 -- Parallelism within partitionwise aggregates
 
-SET max_parallel_workers_per_gather TO 2;
 SET min_parallel_table_scan_size TO '8kB';
 SET parallel_setup_cost TO 0;