As commits b700f96c and 3414099c did for the reloptions test, make
sure VACUUM can always truncate the table as expected.

Back-patch to 12, where vacuum_truncate arrived.

Discussion: https://postgr.es/m/CAD21AoCNoWjYkdEtr%2BVDoF9v__V905AedKZ9iF%3DArgCtrbxZqw%40mail.gmail.com