♻️ Use flutter-io.cn rather than neverssl.com (#2325)

NeverSSL no longer supports H2 headers upgrade without TLS. All
alternatives I've searched are not supported:
- http://www.gstatic.com/generate_204
- http://http.badssl.com/
- http://flutter.dev/

Surprisingly, http://flutter-io.cn/ supports all functionality. 😮

---------

Signed-off-by: Luke Cheng <2258420+chenglu@users.noreply.github.com>
Co-authored-by: Luke Cheng <2258420+chenglu@users.noreply.github.com>

dio_test/lib/src/test/basic_tests.dart

@@ -17,7 +17,7 @@ void basicTests(
   group('basic request', () {
     test(
       'works with non-TLS requests',
-      () => dio.get('http://neverssl.com/'),
+      () => dio.get('http://flutter-io.cn/'),
       testOn: 'vm',
     );