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/application_controller_spec.rb

@@ -49,7 +49,7 @@ describe ApplicationController, type: :controller do
 
     it 'returns account if signed in' do
       account = Fabricate(:account)
-      sign_in(Fabricate(:user, account: account))
+      sign_in(account.user)
       expect(controller.view_context.current_account).to eq account
     end
   end
@@ -164,13 +164,13 @@ describe ApplicationController, type: :controller do
     end
 
     it 'does nothing if user who signed in is not suspended' do
-      sign_in(Fabricate(:user, account: Fabricate(:account, suspended: false)))
+      sign_in(Fabricate(:account, suspended: false).user)
       get 'success'
       expect(response).to have_http_status(200)
     end
 
     it 'redirects to account status page' do
-      sign_in(Fabricate(:user, account: Fabricate(:account, suspended: true)))
+      sign_in(Fabricate(:account, suspended: true).user)
       get 'success'
       expect(response).to redirect_to(edit_user_registration_path)
     end