Adjust diagnostic transformation test to not wait for two buffer notifications

crates/gpui/src/app/test_app_context.rs

@@ -434,7 +434,9 @@ impl<T: Entity> ModelHandle<T> {
             Duration::from_secs(1)
         };
 
+        let executor = cx.background().clone();
         async move {
+            executor.start_waiting();
             let notification = crate::util::timeout(duration, rx.next())
                 .await
                 .expect("next notification timed out");

crates/gpui/src/executor.rs

@@ -876,6 +876,14 @@ impl Background {
             }
         }
     }
+
+    #[cfg(any(test, feature = "test-support"))]
+    pub fn start_waiting(&self) {
+        match self {
+            Self::Deterministic { executor, .. } => executor.start_waiting(),
+            _ => panic!("this method can only be called on a deterministic executor"),
+        }
+    }
 }
 
 impl Default for Background {