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/auth/registrations_controller_spec.rb

@@ -228,7 +228,7 @@ RSpec.describe Auth::RegistrationsController, type: :controller do
     end
 
     it 'does nothing if user already exists' do
-      Fabricate(:user, account: Fabricate(:account, username: 'test'))
+      Fabricate(:account, username: 'test')
       subject
     end
 

spec/controllers/auth/sessions_controller_spec.rb

@@ -37,8 +37,11 @@ RSpec.describe Auth::SessionsController, type: :controller do
     end
 
     context 'with a suspended user' do
+      before do
+        user.account.suspend!
+      end
+
       it 'redirects to home after sign out' do
-        Fabricate(:account, user: user, suspended: true)
         sign_in(user, scope: :user)
         delete :destroy
 
@@ -78,8 +81,8 @@ RSpec.describe Auth::SessionsController, type: :controller do
       end
 
       context 'using a valid email and existing user' do
-        let(:user) do
-          account = Fabricate.build(:account, username: 'pam_user1')
+        let!(:user) do
+          account = Fabricate.build(:account, username: 'pam_user1', user: nil)
           account.save!(validate: false)
           user = Fabricate(:user, email: 'pam@example.com', password: nil, account: account, external: true)
           user