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/accounts_controller_spec.rb

@@ -61,7 +61,7 @@ RSpec.describe Admin::AccountsController, type: :controller do
 
   describe 'GET #show' do
     let(:current_user) { Fabricate(:user, admin: true) }
-    let(:account) { Fabricate(:account, username: 'bob') }
+    let(:account) { Fabricate(:account) }
 
     it 'returns http success' do
       get :show, params: { id: account.id }
@@ -73,7 +73,7 @@ RSpec.describe Admin::AccountsController, type: :controller do
     subject { post :memorialize, params: { id: account.id } }
 
     let(:current_user) { Fabricate(:user, admin: current_user_admin) }
-    let(:account) { Fabricate(:account, user: user) }
+    let(:account) { user.account }
     let(:user) { Fabricate(:user, admin: target_user_admin) }
 
     context 'when user is admin' do
@@ -125,7 +125,7 @@ RSpec.describe Admin::AccountsController, type: :controller do
     subject { post :enable, params: { id: account.id } }
 
     let(:current_user) { Fabricate(:user, admin: admin) }
-    let(:account) { Fabricate(:account, user: user) }
+    let(:account) { user.account }
     let(:user) { Fabricate(:user, disabled: true) }
 
     context 'when user is admin' do