unionparallel test sometimes fails

Running on a single node, running the union parallel test in a loop sometimes results in failure, specifically 10_1_
join.req

```
10:25:47> cdb2sql -s --cdb2cfg /home/schandra107/comdb2/tests/test_69339/unionparallel69339/comdb2db.cfg unionparallel69339 default - < 10_1_join.req > 10_1_join.req.res 2>&1
10:25:47> ^^^^^^^^^^^^
10:25:47> The above testcase (10_1_join.req) has failed!!!
10:25:47>
10:25:47> Use 'diff ' to see why:
10:25:47> > diff /home/schandra107/comdb2/tests/test_69339/union_parallel.test/{10_1_join.req.exp,10_1_join.req.res}
10:25:47>
10:25:47> 72c72
10:25:47> < (a=1)
10:25:47> ---
10:25:47> > (a=2)
10:25:47> 77d76
10:25:47> <
10:25:47>
Duration 23 seconds
```