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/settings/deletes_controller_spec.rb

@@ -17,7 +17,7 @@ describe Settings::DeletesController do
       end
 
       context 'when suspended' do
-        let(:user) { Fabricate(:user, account_attributes: { username: 'alice', suspended_at: Time.now.utc }) }
+        let(:user) { Fabricate(:user, account_attributes: { suspended_at: Time.now.utc }) }
 
         it 'returns http forbidden' do
           get :show
@@ -64,7 +64,7 @@ describe Settings::DeletesController do
         end
 
         context 'when suspended' do
-          let(:user) { Fabricate(:user, account_attributes: { username: 'alice', suspended_at: Time.now.utc }) }
+          let(:user) { Fabricate(:user, account_attributes: { suspended_at: Time.now.utc }) }
 
           it 'returns http forbidden' do
             expect(response).to have_http_status(403)