[9.x] Add tests for session JSON serialization

[Krisell]

This adds tests for the new session JSON serialization option added in Laravel 9 (#40595).

The added tests does not duplicate all test cases for serialize/unserialize, but they do test both reading and writing to the session.

[Krisell]

Should the new JSON serialization option be added to the documentation (e.g. here https://laravel.com/docs/9.x/session#configuration) or should the serialization option perhaps be mentioned in the config/session.php template?

[taylorotwell]

@Krisell JSON serialization is totally broken for view error bag sharing and should probably be removed.

[Krisell]

@taylorotwell I did see your comment here now, and the related issue: laravel/laravel#5796 (comment)

Has your suggested solution there been attempted?

I think using JSON is preferred for several reasons, as described by you in laravel/laravel#5787 and also for the smaller payload size.