Fix flakiness in the pg_visibility VM-only vacuum test by using a temporary table.

Fix flakiness in the pg_visibility VM-only vacuum test by using a temporary table.

The test relies on VACUUM being able to mark a page all-visible, but
this can fail when autovacuum in other sessions prevents the visibility
horizon from advancing. Making the test table temporary isolates its
horizon from other sessions, including catalog table vacuums, ensuring
reliable test behavior.

Reported-by: Alexander Lakhin <exclusion@gmail.com>
Author: Kirill Reshke <reshkekirill@gmail.com>
Reviewed-by: Melanie Plageman <melanieplageman@gmail.com>
Discussion: https://postgr.es/m/2b09fba6-6b71-497a-96ef-a6947fcc39f6%40gmail.com