Stabilize plancache test against on-access VM setting

Stabilize plancache test against on-access VM setting

Since b46e1e54d07 allowed setting the VM on-access and 378a21618 set
pd_prune_xid on INSERT, the testing of generic/custom plans in
src/test/regress/sql/plancache.sql was destabilized.

One of the queries of test_mode could have set the pages all-visible and
if autovacuum/autoanalyze ran and updated pg_class.relallvisible, it
would affect whether we got an index-only or sequential scan.

Preclude this by disabling autovacuum and autoanalyze for test_mode and
carefully sequencing when ANALYZE is run.

Reported-by: Alexander Lakhin <exclusion@gmail.com>
Author: Melanie Plageman <melanieplageman@gmail.com>
Discussion: https://postgr.es/m/71277259-264e-4983-a201-938b404049d7%40gmail.com