Reload Tributaries

add_active_tributary writes the spec to disk before it returns, so even if the
VecDeque it pushes to isn't popped, the tributary will still be loaded on boot.

coordinator/src/tests/tributary/chain.rs

@@ -61,7 +61,9 @@ pub fn new_spec<R: RngCore + CryptoRng>(
       .unwrap(),
   };
 
-  TributarySpec::new(serai_block, start_time, set, set_data)
+  let res = TributarySpec::new(serai_block, start_time, set, set_data);
+  assert_eq!(TributarySpec::read::<&[u8]>(&mut res.serialize().as_ref()).unwrap(), res);
+  res
 }
 
 pub async fn new_tributaries(