Rails 6 removes hidden input named "utf8" to forms.

[lcreid]

Rails 6 removes the hidden input field named "utf8", which breaks all the tests. See rails/rails#32125.

A note about possible solutions: form_with changed the output of helpers compared to form_for, and testing for both quickly became very complicated. DRY solutions to supporting different expected strings quickly become hard to understand. Non-DRY solutions require maintenance of a lot of boilerplate code. We didn't find a good balance in the case of form_with.

Part of issue #503.