[sled-storage] Automatically clean up test zpools, storage

[smklein]

StorageManagerTestHarness is a utility which allows sled agent tests to use real zpools and datasets atop temporary file-backed storage. This provides a decent test fidelity for storage-based parts of the sled agent.

This harness previously took a liberal attitude towards zpool creation, but a conservative perspective on their deletion: rather than automatically destroying zpools on test failure, it would print commands for "how to delete these zpools yourself". This tendency was mostly borne out of fear: deleting zpools is a dangerous operation which should be done with caution.

When working with the StorageManagerTestHarness, especially while iterating on tests with many expected failures, I've decided this is an Enormous Pain In The Butt. This PR changes the aforementioned tendency, and just attempts to delete all provisioned zpools we created in our tests. If we fail to do so, we'll then log an error message and let the user take responsibility for manual cleanup.

[smklein]

Made this while working on #6782. IMO it's a nice quality-of-life improvement for development.

[andrewjstone]

Very nice QOL improvement indeed.