Refactor and improve tests (#17386)

* Change account and user fabricators to simplify and improve tests

- `Fabricate(:account)` implicitly fabricates an associated `user` if
  no `domain` attribute is given (an account with `domain: nil` is
  considered a local account, but no user record was created), unless
  `user: nil` is passed
- `Fabricate(:account, user: Fabricate(:user))` should still be possible
  but is discouraged.

* Fix and refactor tests

- avoid passing unneeded attributes to `Fabricate(:user)` or
  `Fabricate(:account)`
- avoid embedding `Fabricate(:user)` into a `Fabricate(:account)` or the other
  way around
- prefer `Fabricate(:user, account_attributes: …)` to
  `Fabricate(:user, account: Fabricate(:account, …)`
- also, some tests were using remote accounts with local user records, which is
  not representative of production code.

spec/controllers/admin/change_email_controller_spec.rb

@@ -11,10 +11,9 @@ RSpec.describe Admin::ChangeEmailsController, type: :controller do
 
   describe "GET #show" do
     it "returns http success" do
-      account = Fabricate(:account)
-      user = Fabricate(:user, account: account)
+      user = Fabricate(:user)
 
-      get :show, params: { account_id: account.id }
+      get :show, params: { account_id: user.account.id }
 
       expect(response).to have_http_status(200)
     end
@@ -26,12 +25,11 @@ RSpec.describe Admin::ChangeEmailsController, type: :controller do
     end
 
     it "returns http success" do
-      account = Fabricate(:account)
-      user = Fabricate(:user, account: account)
+      user = Fabricate(:user)
 
       previous_email = user.email
 
-      post :update, params: { account_id: account.id, user: { unconfirmed_email: 'test@example.com' } }
+      post :update, params: { account_id: user.account.id, user: { unconfirmed_email: 'test@example.com' } }
 
       user.reload
 
@@ -41,7 +39,7 @@ RSpec.describe Admin::ChangeEmailsController, type: :controller do
 
       expect(UserMailer).to have_received(:confirmation_instructions).with(user, user.confirmation_token, { to: 'test@example.com' })
 
-      expect(response).to redirect_to(admin_account_path(account.id))
+      expect(response).to redirect_to(admin_account_path(user.account.id))
     end
   end
 end