ZTS: tests for dedup legacy/FDT tables

Very basic coverage to make sure things appear to work, have the right
format on disk, and pool upgrades and mixed table types work as
expected.

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Sponsored-by: Klara, Inc.
Sponsored-by: iXsystems, Inc.
Closes #15892