Add specs for adding-to/clear has_many collections’s behavior on `updated_at`

There are behaviors mentioned in #17161 that:

1. are not documented properly, and
2. don't have specs

This PR addresses the spec absence. For `has_many` collections,

1. addition (`<<`) should update the associated object's `updated_at` (if any)
2. `.clear` (which, depending on `options[:dependent]`) calls `delete_all`, `destroy_all`, or just nullifying the associated object's foreign key.