Unflake page test that was causing us a lot of CI flakes (#625)

Author: kblok

lib/PuppeteerSharp.Tests/PageTests/WaitForNavigationTests.cs

@@ -162,8 +162,10 @@ public async Task ShouldWorkWhenSubframeIssuesWindowStop()
                 }
             };
             await frameNavigatedTaskSource.Task;
-            var task = frame.EvaluateFunctionAsync("() => window.stop()");
-            await navigationTask;
+            await Task.WhenAll(
+                frame.EvaluateFunctionAsync("() => window.stop()"),
+                navigationTask
+            );
         }
     }
 }