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

src/test/regress/expected/vacuum.out

@@ -165,19 +165,19 @@ VACUUM (INDEX_CLEANUP FALSE) vaccluster;
 VACUUM (INDEX_CLEANUP AUTO) vactst; -- index cleanup option is ignored if no indexes
 VACUUM (INDEX_CLEANUP FALSE, FREEZE TRUE) vaccluster;
 -- TRUNCATE option
-CREATE TABLE vac_truncate_test(i INT NOT NULL, j text)
+CREATE TEMP TABLE vac_truncate_test(i INT NOT NULL, j text)
 	WITH (vacuum_truncate=true, autovacuum_enabled=false);
 INSERT INTO vac_truncate_test VALUES (1, NULL), (NULL, NULL);
 ERROR:  null value in column "i" of relation "vac_truncate_test" violates not-null constraint
 DETAIL:  Failing row contains (null, null).
-VACUUM (TRUNCATE FALSE) vac_truncate_test;
+VACUUM (TRUNCATE FALSE, DISABLE_PAGE_SKIPPING) vac_truncate_test;
 SELECT pg_relation_size('vac_truncate_test') > 0;
  ?column? 
 ----------
  t
 (1 row)
 
-VACUUM vac_truncate_test;
+VACUUM (DISABLE_PAGE_SKIPPING) vac_truncate_test;
 SELECT pg_relation_size('vac_truncate_test') = 0;
  ?column? 
 ----------

src/test/regress/sql/vacuum.sql

@@ -147,12 +147,12 @@ VACUUM (INDEX_CLEANUP AUTO) vactst; -- index cleanup option is ignored if no ind
 VACUUM (INDEX_CLEANUP FALSE, FREEZE TRUE) vaccluster;
 
 -- TRUNCATE option
-CREATE TABLE vac_truncate_test(i INT NOT NULL, j text)
+CREATE TEMP TABLE vac_truncate_test(i INT NOT NULL, j text)
 	WITH (vacuum_truncate=true, autovacuum_enabled=false);
 INSERT INTO vac_truncate_test VALUES (1, NULL), (NULL, NULL);
-VACUUM (TRUNCATE FALSE) vac_truncate_test;
+VACUUM (TRUNCATE FALSE, DISABLE_PAGE_SKIPPING) vac_truncate_test;
 SELECT pg_relation_size('vac_truncate_test') > 0;
-VACUUM vac_truncate_test;
+VACUUM (DISABLE_PAGE_SKIPPING) vac_truncate_test;
 SELECT pg_relation_size('vac_truncate_test') = 0;
 VACUUM (TRUNCATE FALSE, FULL TRUE) vac_truncate_test;
 DROP TABLE vac_truncate_test;