Add more DDT tests

The new Fast Dedup feature has a lot of moving parts, and only some of
them have tests. We have some tests for prefetch and quota, and a
generic ZAP shrinking test, but we don't have anything for the pruning
command or specific to DDT zap shrinking. Here we add a couple small new
tests for zpool ddtprune and DDT-specific ZAP shrinking.

Sponsored-by: Klara, Inc.
Sponsored-by: iXsystems, Inc.
Reviewed-by: Allan Jude <allan@klarasystems.com>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Paul Dagnelie <paul.dagnelie@klarasystems.com>
Closes #17049