Proper usage for shards_watcher

Signed-off-by: Andrey Berezin <a.berezin@yadro.com>

src/frostfs_testlib/storage/controllers/shards_watcher.py

@@ -99,6 +99,7 @@ class ShardsWatcher:
             endpoint=self.storage_node.get_control_endpoint(),
             wallet=self.storage_node.get_remote_wallet_path(),
             wallet_password=self.storage_node.get_wallet_password(),
+            json_mode=True,
         )
 
         return json.loads(response.stdout.split(">", 1)[1])