Use OFFSET 0 instead of ORDER BY to stop subquery pullup

Use OFFSET 0 instead of ORDER BY to stop subquery pullup

b762fed64 recently changed this test to prevent subquery pullup to allow
us to test Memoize with lateral_vars.  As pointed out by Tom Lane, OFFSET
0 is our standard way of preventing subquery pullups, so do it that way
instead.

Discussion: https://postgr.es/m/2144818.1674517061@sss.pgh.pa.us
Backpatch-through: 14, same as b762fed64