Subquery multiple return value test unstable

qp_union_intersect,

In qp_union_intersect_optimizer.out line 1736，the sql
```
set optimizer_segments=2;
ANALYZE dml_union_r, dml_union_s;
UPDATE dml_union_r SET a = ( SELECT a FROM dml_union_r UNION ALL SELECT a|1724 -- ERROR: one or more assertions failed FROM dml_union_s);
```
should always get the result:
```
ERROR: more than one row returned by a subquery used as an expression (seg2 slice1 10.138.0.30:7004 pid=3108610)
```
but we get result sometime:
```
ERROR: one or more assertions failed
DETAIL: Expected no more than one row to be returned by expression
```

Start with the qp_union_intersect test and qp_union_intersect_optimizer.out around line 1736, then reproduce the UPDATE with optimizer_segments=2 and the ANALYZE steps shown. Investigate why the same multiple-row subquery sometimes produces an assertion failure instead of the reported more-than-one-row error; done means the test consistently reports the expected error.

Written by the indexing model from the issue text.