Fix flaky test_block_mailing_list() (#3733)

Seems like consume_events() didn't work properly, i.e. in some cases it
didn't see the latest events and failed to consume them. So, the
IncomingMsg event from receiving DC_MAILINGLIST stayed in the events
channel, which made this fail:

```rust
        // Check that no notification is displayed for blocked mailing list message.
        while let Ok(event) = t.evtracker.try_recv() {
            assert!(!matches!(event.typ, EventType::IncomingMsg { .. }));
        }
```

Fix it by explicitly waiting for the first IncomingMsg event.

src/receive_imf.rs

@@ -3149,6 +3149,7 @@ Hello mailinglist!\r\n"
             .unwrap();
 
         receive_imf(&t.ctx, DC_MAILINGLIST, false).await.unwrap();
+        t.evtracker.wait_next_incoming_message().await;
         let chats = Chatlist::try_load(&t.ctx, 0, None, None).await.unwrap();
         assert_eq!(chats.len(), 1);
         let chat_id = chats.get_chat_id(0).unwrap();
@@ -3161,7 +3162,6 @@ Hello mailinglist!\r\n"
         let chats = Chatlist::try_load(&t.ctx, 0, None, None).await.unwrap();
         assert_eq!(chats.len(), 0); // Test that the message disappeared
 
-        t.evtracker.consume_events();
         receive_imf(&t.ctx, DC_MAILINGLIST2, false).await.unwrap();
 
         // Check that no notification is displayed for blocked mailing list message.

src/test_utils.rs

@@ -876,9 +876,10 @@ impl EventTracker {
         .await
     }
 
-    /// Consumes all pending events.
-    pub fn consume_events(&self) {
-        while self.try_recv().is_ok() {}
+    /// Wait for the next IncomingMsg event.
+    pub async fn wait_next_incoming_message(&self) {
+        self.get_matching(|evt| matches!(evt, EventType::IncomingMsg { .. }))
+            .await;
     }
 }