pgsql: Try to stabilize vacuum test.

Try to stabilize vacuum test.

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

Branch
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REL_14_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/cd3a5055f952ca2811e63d86558dd3e1f4147f62

Modified Files
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src/test/regress/expected/vacuum.out | 6 +++---
src/test/regress/sql/vacuum.sql      | 6 +++---
2 files changed, 6 insertions(+), 6 deletions(-)