Readd config.cache_classes on test env and remove with_model dep

[waiting-for-dev]

Rails' config setting cache_classes was recently set to false in the
dummy app (see
ef9b5ab).
That was due to a misunderstanding, as Rails recommends it be true
except when spring is present. From
https://guides.rubyonrails.org/configuring.html#config-cache-classes:

Controls whether or not application classes and modules should be
reloaded if they change. When the cache is enabled (true), reloading
will not occur. Defaults to false in the development environment, and
true in production. In the test environment, the default is false if
Spring is installed, true otherwise.

That wasn't such a big deal until now. However, if we try to use the new
Ruby 3.1 in conjunction with cache_classes in the test environment,
many errors related to STI classes not being found pop out.

However, having the correct false value on Rails 7 raises an error
sourced on the with_model gem. See
Casecommons/with_model#35.

Because of that, we remove the with_model dependency so that our path
to support Ruby 3.1 is unblocked. We take different decisions to remove
the occurrences of with_model usage on Solidus:

• Tests on Spree::Api::ResourceController. That controller was only
  used on Spree::Api::UsersController. Therefore, it was no longer a
  valid abstraction:

  • We removed the tests altogether.
  • We moved the missing pieces to UsersController.
  • We removed the soft deletable specific test and added one to UsersController.
  • We marked ResourceController as deprecated (not removing it just in case some store uses it).

• Tests on Spree::Admin::ResourceController, Spree::CalculatedAdjustements,
  Spree::Validations::DbMaximumLengthValidtor &
  Spree::WalletPaymentSource: we use a custom logic for the same
  behavior provided by with_model.

• Tests on Spree::SoftDeletable: we use Spree::Product as a fixture.

[kennyadsl]

I like having a dependency less, and those Widget tests were always confusing to me.

[jarednorman]

I'm vaguely apprehensive that extensions or stores might be using this, but it's a pretty easy change for them to switch off. Seems good.

[jarednorman]

"I didn't even know there was a Spree::Api::ResourceController."–@benjaminwil

[jarednorman]

There's a typo in the commit message/PR name: "Readd"

[waiting-for-dev]

There's a typo in the commit message/PR name: "Readd"

@jarednorman, hmm... I used readd as in re+add, i.e., add again 🙂 Does it sound too weird? Not an English native speaker, so I'll trust your judgment more than mine one thousand times 😉

[waiting-for-dev]

There's a typo in the commit message/PR name: "Readd"

@jarednorman, hmm... I used readd as in re+add, i.e., add again slightly_smiling_face Does it sound too weird? Not an English native speaker, so I'll trust your judgment more than mine one thousand times wink

Going ahead with the merge to not unblock support for Ruby 2.7 🙂