Attempt to fix newly added Memoize regression test

Attempt to fix newly added Memoize regression test

Both drongo and fairywren seem not to like a new regression test added
by 2cca95e17.  These machines show a different number of actual rows in
the EXPLAIN ANALYZE output.  Since the number of actual rows is divided by
the number of loops, I suspect this might be due to some platform
dependant rounding behavior as the total row count is 5 and the number of
loops is 2.  drongo and fairywren seem to be calculating that 5.0 / 2.0 is
3, whereas most other machines think the answer is 2.

Here we tweak the test query's WHERE clause so it's 4.0 / 2.0 instead.
There shouldn't be too much wiggle room for platform dependant-behavior to
be a factor with those numbers.

Reported-by: Tom Lane
Discussion: https://postgr.es/m/1035225.1706301718%40sss.pgh.pa.us