Avoid possible regression test instability in timestamp.sql.

Concurrent autovacuum could result in a change in the order of the
live rows in timestamp_tbl.  While this would not happen with the
default autovacuum parameters, it's fairly easy to hit if
autovacuum_vacuum_threshold is made small enough to allow autovac
to decide to process this table.  That's a stumbling block for trying
to exercise autovacuum aggressively using the core regression tests.

To fix, replace an unqualified DELETE with a TRUNCATE.  There's a
similar DELETE just above (and no order-sensitive queries between),
so this doesn't lose any test coverage and might indeed be argued
to improve it.

Discussion: https://postgr.es/m/17428.1555348950@sss.pgh.pa.us

src/test/regress/expected/timestamp.out

@@ -74,7 +74,7 @@ SELECT count(*) AS two FROM TIMESTAMP_TBL WHERE d1 = timestamp(2) without time z
 (1 row)
 
 COMMIT;
-DELETE FROM TIMESTAMP_TBL;
+TRUNCATE TIMESTAMP_TBL;
 -- Special values
 INSERT INTO TIMESTAMP_TBL VALUES ('-infinity');
 INSERT INTO TIMESTAMP_TBL VALUES ('infinity');

src/test/regress/sql/timestamp.sql

@@ -44,7 +44,7 @@ SELECT pg_sleep(0.1);
 SELECT count(*) AS two FROM TIMESTAMP_TBL WHERE d1 = timestamp(2) without time zone 'now';
 COMMIT;
 
-DELETE FROM TIMESTAMP_TBL;
+TRUNCATE TIMESTAMP_TBL;
 
 -- Special values
 INSERT INTO TIMESTAMP_TBL VALUES ('-infinity');