Improve stability fix for partition_aggregate test.

 Improve stability fix for partition_aggregate test.
 
 Instead of disabling autovacuum on these test tables, adjust the
 partition boundaries so that the child partitions are not all the
 same size.  That should cause the planner to use a predictable
 ordering of the per-partition scan nodes even in cases where
 autovacuum causes the rowcount estimates to be off a bit.
 Moreover, this also lets these tests show that the planner does
 properly order the tables in descending size order, something
 that wasn't being proven before.
 
 The pagg_tab1 and pagg_tab2 partitions are still all the same
 size, but that should be fine, because those tables are so small
 that (1) autovacuum won't fire on them, and (2) even if it did,
 it couldn't change the reltuples value --- with only one page,
 it can't see just part of the relation.
 
 Discussion: https://postgr.es/m/24467.1585838693@sss.pgh.pa.us