Improve expectation in unit test

Author: mvanbeusekom

packages/webview_flutter/webview_flutter_android/test/webview_android_widget_test.dart

@@ -888,8 +888,7 @@ void main() {
               });
 
       webViewClient.urlLoading(MockWebView(), 'https://flutter.dev');
-      await completer.future;
-      expect(completer.isCompleted, isTrue);
+      expect(completer.future, completes);
     });
 
     test(
@@ -983,8 +982,7 @@ void main() {
               });
 
       webViewClient.requestLoading(MockWebView(), mockRequest);
-      await completer.future;
-      expect(completer.isCompleted, isTrue);
+      expect(completer.future, completes);
     });
 
     test(