test: Expect non-null disk state when usage is not supported

Previously, when usage was not supported, we returned null root disk and thus did not show disk total size, even when it was possible. Now we represent not supporting showing disk usage with usage=-1.

Signed-off-by: hamistao <[email protected]>

test/suites/storage.sh

@@ -931,8 +931,8 @@ EOF
     # Disable quotas. The usage should be 0.
     # shellcheck disable=SC2031
     btrfs quota disable "${LXD_DIR}/storage-pools/${pool_name}"
-    usage=$(lxc query /1.0/instances/c1/state | jq '.disk.root')
-    [ "${usage}" = "null" ]
+    # Usage -1 indicates the driver does not support getting instance usage.
+    [ "$(lxc query /1.0/instances/c1/state | jq '.disk.root.usage')" = "-1" ]
 
     # Enable quotas. The usage should then be > 0.
     # shellcheck disable=SC2031