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/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;
     }
 }