core: Run async tasks until stalled

Previously it used run which tries to run until completion, but we can't do
that if we are testing socket behavior.

core/src/backend/navigator.rs

@@ -289,7 +289,7 @@ impl NullExecutor {
     }
 
     pub fn run(&mut self) {
-        self.0.run();
+        self.0.run_until_stalled();
     }
 }