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/services/fan_out_on_write_service_spec.rb

@@ -3,9 +3,9 @@ require 'rails_helper'
 RSpec.describe FanOutOnWriteService, type: :service do
   let(:last_active_at) { Time.now.utc }
 
-  let!(:alice) { Fabricate(:user, current_sign_in_at: last_active_at, account: Fabricate(:account, username: 'alice')).account }
-  let!(:bob)   { Fabricate(:user, current_sign_in_at: last_active_at, account: Fabricate(:account, username: 'bob')).account }
-  let!(:tom)   { Fabricate(:user, current_sign_in_at: last_active_at, account: Fabricate(:account, username: 'tom')).account }
+  let!(:alice) { Fabricate(:user, current_sign_in_at: last_active_at).account }
+  let!(:bob)   { Fabricate(:user, current_sign_in_at: last_active_at, account_attributes: { username: 'bob' }).account }
+  let!(:tom)   { Fabricate(:user, current_sign_in_at: last_active_at).account }
 
   subject { described_class.new }