Fix client state test cleanup (#1864)

Closes #1862
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### This PR:
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Adds a cleanup step to the
`test_process_client_handling_stream_request_latest_voters_snapshot`
function in order to ensure that a `panic!` can be avoided in the test
cleanup.

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node-metrics/src/service/client_state/mod.rs

@@ -1308,7 +1308,7 @@ pub mod tests {
         let (internal_client_message_sender, internal_client_message_receiver) = mpsc::channel(1);
         let (server_message_sender_1, mut server_message_receiver_1) = mpsc::channel(1);
         let (server_message_sender_2, mut server_message_receiver_2) = mpsc::channel(1);
-        let _process_internal_client_message_handle = InternalClientMessageProcessingTask::new(
+        let mut process_internal_client_message_handle = InternalClientMessageProcessingTask::new(
             internal_client_message_receiver,
             data_state,
             client_thread_state,
@@ -1359,6 +1359,10 @@ pub mod tests {
                 voters_1, voters_2
             ]))),
         );
+
+        if let Some(task_handle) = process_internal_client_message_handle.task_handle.take() {
+            assert_eq!(task_handle.cancel().await, None);
+        }
     }
 
     #[async_std::test]