chore: organize imports

test/widget_test.dart

@@ -5,9 +5,13 @@
 // gestures. You can also use WidgetTester to find child widgets in the widget
 // tree, read text, and verify that the values of widget properties are correct.
 
+// Flutter imports:
 import 'package:flutter/material.dart';
+
+// Package imports:
 import 'package:flutter_test/flutter_test.dart';
 
+// Project imports:
 import 'package:openlib/main.dart';
 
 void main() {