Add more DDT tests

[pcd1193182]

Motivation and Context

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. We also don't have anything for the dedup log, but I added a test for that in #17038 .

Description

We add a couple small new tests for zpool ddtprune and DDT-specific ZAP shrinking.

How Has This Been Tested?

Ran the ZTS with the new tests, they passed

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.