Minor regression test fail on Valgrind

We get this regression test diff on Valgrind tests. It seems that this is not as a bug, still it would be better to have consistent results.

```
--- /home/ubuntu/citus/src/test/regress/expected/adaptive_executor.out 2019-07-07 01:05:05.555179365 +0000
+++ /home/ubuntu/citus/src/test/regress/results/adaptive_executor.out 2019-07-07 02:46:25.811991773 +0000
@@ -41,16 +41,16 @@
-------
2
(1 row)

SELECT sum(result::bigint) FROM run_command_on_workers($$
SELECT count(*) FROM pg_stat_activity
WHERE pid <> pg_backend_pid() AND query LIKE '%8010090%'
$$);
sum
-----
- 4
+ 2
(1 row)

END;
DROP SCHEMA adaptive_executor CASCADE;
NOTICE: drop cascades to table test

======================================================================
```