ZTS: Written props test fails with 4k disks

ZTS: Written props test fails with 4k disks

With 4k disks, this test will fail in the last section because the
expected human readable value of 20.0M is reported as 20.1M. Rather than
use the human readable property, switch to the parsable property and
verify that the values are reasonably close.

Reviewed-by: Igor Kozhukhov <igor@dilos.org>
Reviewed-by: Ryan Moeller <ryan@ixsystems.com>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: John Kennedy <john.kennedy@delphix.com>
Closes #9477