[sled-storage] Use test-configured mountpoint root for datasets (#7110)

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:


https://github.com/oxidecomputer/omicron/blob/48790e52a9b76ec91c315ca44f5c462d639e62f7/sled-storage/src/config.rs#L33-L35

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.

sled-storage/src/manager.rs

@@ -955,8 +955,8 @@ impl StorageManager {
             err: None,
         };
 
-        let mountpoint_path =
-            config.name.mountpoint(ZPOOL_MOUNTPOINT_ROOT.into());
+        let mountpoint_root = &self.resources.disks().mount_config().root;
+        let mountpoint_path = config.name.mountpoint(mountpoint_root);
         let details = DatasetCreationDetails {
             zoned: config.name.dataset().zoned(),
             mountpoint: Mountpoint::Path(mountpoint_path),