Attempt to stabilize new window agg regression test

Attempt to stabilize new window agg regression test

This test was recently added in 3900a02c9.  It appears to be unstable in
regards to the join order presumably due to the relations at either side
of the join being equal in side.  Here we add a qual to make one of them
smaller so the planner is more likely to choose to hash the smaller of the
two.

Reported-by: Nathan Bossart, Tom Lane
Discussion: https://www.postgr.es/m/20230803235403.GC1238296@nathanxps13