Disable Postgres parallelism by default in tests

src/test/regress/expected/multi_partitioning.out

@@ -1591,20 +1591,18 @@ SELECT * FROM partitioning_hash_test JOIN partitioning_hash_join_test USING (id,
    Tasks Shown: One of 4
    ->  Task
          Node: host=localhost port=xxxxx dbname=regression
-         ->  Gather
-               Workers Planned: 2
-               ->  Parallel Hash Join
-                     Hash Cond: ((partitioning_hash_join_test.id = partitioning_hash_test_1.id) AND (partitioning_hash_join_test.subid = partitioning_hash_test_1.subid))
-                     ->  Parallel Append
-                           ->  Parallel Seq Scan on partitioning_hash_join_test_0_1660133 partitioning_hash_join_test
-                           ->  Parallel Seq Scan on partitioning_hash_join_test_1_1660137 partitioning_hash_join_test_1
-                           ->  Parallel Seq Scan on partitioning_hash_join_test_2_1660141 partitioning_hash_join_test_2
-                     ->  Parallel Hash
-                           ->  Parallel Append
-                                 ->  Parallel Seq Scan on partitioning_hash_test_1_1660020 partitioning_hash_test_1
-                                 ->  Parallel Seq Scan on partitioning_hash_test_0_1660016 partitioning_hash_test
-                                 ->  Parallel Seq Scan on partitioning_hash_test_2_1660032 partitioning_hash_test_2
-(18 rows)
+         ->  Hash Join
+               Hash Cond: ((partitioning_hash_join_test.id = partitioning_hash_test.id) AND (partitioning_hash_join_test.subid = partitioning_hash_test.subid))
+               ->  Append
+                     ->  Seq Scan on partitioning_hash_join_test_0_1660133 partitioning_hash_join_test
+                     ->  Seq Scan on partitioning_hash_join_test_1_1660137 partitioning_hash_join_test_1
+                     ->  Seq Scan on partitioning_hash_join_test_2_1660141 partitioning_hash_join_test_2
+               ->  Hash
+                     ->  Append
+                           ->  Seq Scan on partitioning_hash_test_0_1660016 partitioning_hash_test
+                           ->  Seq Scan on partitioning_hash_test_1_1660020 partitioning_hash_test_1
+                           ->  Seq Scan on partitioning_hash_test_2_1660032 partitioning_hash_test_2
+(16 rows)
 
 -- set partition-wise join on and parallel to off
 SELECT success FROM run_command_on_workers('alter system set max_parallel_workers_per_gather = 0');

src/test/regress/pg_regress_multi.pl

@@ -320,6 +320,10 @@ if (-e $hll_control)
 }
 push(@pgOptions, '-c', "shared_preload_libraries=${sharedPreloadLibraries}");
 
+# Avoid parallelism to stabilize explain plans
+push(@pgOptions, '-c', "max_parallel_workers_per_gather=0");
+
+# Allow CREATE SUBSCRIPTION to work
 push(@pgOptions, '-c', "wal_level=logical");
 
 # Citus options set for the tests