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/confirmations_controller_spec.rb

@@ -9,9 +9,8 @@ RSpec.describe Admin::ConfirmationsController, type: :controller do
 
   describe 'POST #create' do
     it 'confirms the user' do
-      account = Fabricate(:account)
-      user = Fabricate(:user, confirmed_at: false, account: account)
-      post :create, params: { account_id: account.id }
+      user = Fabricate(:user, confirmed_at: false)
+      post :create, params: { account_id: user.account.id }
 
       expect(response).to redirect_to(admin_accounts_path)
       expect(user.reload).to be_confirmed
@@ -32,10 +31,9 @@ RSpec.describe Admin::ConfirmationsController, type: :controller do
   end
 
   describe 'POST #resernd' do
-    subject { post :resend, params: { account_id: account.id } }
+    subject { post :resend, params: { account_id: user.account.id } }
 
-    let(:account) { Fabricate(:account) }
-    let!(:user) { Fabricate(:user, confirmed_at: confirmed_at, account: account) }
+    let!(:user) { Fabricate(:user, confirmed_at: confirmed_at) }
 
     before do
       allow(UserMailer).to receive(:confirmation_instructions) { double(:email, deliver_later: nil) }