Fix flaky 0_stateless/02784_parallel_replicas_automatic_decision

- avoid max_threads randomzation

tests/queries/0_stateless/02784_parallel_replicas_automatic_decision.sh

@@ -53,6 +53,7 @@ function run_query_with_pure_parallel_replicas () {
         --allow_experimental_parallel_reading_from_replicas 1 \
         --parallel_replicas_for_non_replicated_merge_tree 1 \
         --parallel_replicas_min_number_of_rows_per_replica "$2" \
+        --max_threads 5 \
     |& grep "It is enough work for" | awk '{ print substr($7, 2, length($7) - 2) "\t" $20 " estimated parallel replicas" }'
 }