[sled-storage] Use test-configured mountpoint root for datasets

[smklein]

This change should only impact tests.

The "mountpoint" configuration, by default, uses the ZPOOL_MOUNTPOINT_ROOT as a root path where datasets
may be located:

omicron/sled-storage/src/config.rs

Lines 33 to 35 in 48790e5

	root: Utf8PathBuf::from(
	illumos_utils::zpool::ZPOOL_MOUNTPOINT_ROOT,
	),

In prod, this value is "/". Under tests, however, we may want more isolation between datasets, hence this configuration option.

This PR respects that configuration option when configuring datasets. This will be relevant for Sled Agent tests, and otherwise effectively acts as a no-op.

[smklein]

Discovered this while working on #6782 - after this PR, it's possible to use the "ensure dataset" calls as a part of the StorageManagerTestHarness.

[andrewjstone]

Nice find!