Soon-to-be-broken regression test case

The last test case in select_parallel.sql, added in commit dc1057fc,
currently generates a plan like this:

CREATE VIEW tenk1_vw_sec WITH (security_barrier) AS SELECT * FROM tenk1;
EXPLAIN (COSTS OFF)
SELECT 1 FROM tenk1_vw_sec WHERE EXISTS (SELECT 1 WHERE unique1 = 0);
QUERY PLAN
-------------------------------------------------------------------
Subquery Scan on tenk1_vw_sec
Filter: (alternatives: SubPlan 1 or hashed SubPlan 2)
-> Gather
Workers Planned: 4
-> Parallel Index Only Scan using tenk1_unique1 on tenk1
SubPlan 1
-> Result
One-Time Filter: (tenk1_vw_sec.unique1 = 0)
SubPlan 2
-> Result
(10 rows)

I have been fooling around with a patch to allow pull-up of sub-selects
that lack any FROM, along the lines discussed in
https://www.postgresql.org/message-id/15944.1521127664@sss.pgh.pa.us
I find that it is smart enough to reduce that EXISTS to a plain
expression, yielding

QUERY PLAN
----------------------------------------------------
Subquery Scan on tenk1_vw_sec
-> Index Only Scan using tenk1_unique1 on tenk1
Index Cond: (unique1 = 0)
(3 rows)

While that's obviously a far better plan, it does not meet this test
case's stated goal of testing the interaction of subqueries with
parallel query. Could you suggest a less trivial subquery that will
still do what you intended?

regards, tom lane