pgsql: Use pg_pwrite() in pg_test_fsync.

Use pg_pwrite() in pg_test_fsync.

For consistency with the PostgreSQL behavior this test program is
intended to simulate, use pwrite() instead of lseek() + write().

Also fix the final "non-sync" test, which was opening and closing the
file for every write.

Discussion: https://postgr.es/m/CA%2BhUKGJjjid2BJsvjMALBTduo1ogdx2SPYaTQL3wAy8y2hc4nw%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/2c8b42b50df6cc5ba6987c400a857d6bbfdf0300

Modified Files
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src/bin/pg_test_fsync/pg_test_fsync.c | 51 +++++++++++++++++++----------------
1 file changed, 28 insertions(+), 23 deletions(-)