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/mailers/notification_mailer_spec.rb

@@ -1,7 +1,7 @@
 require "rails_helper"
 
 RSpec.describe NotificationMailer, type: :mailer do
-  let(:receiver)       { Fabricate(:user, account: Fabricate(:account, username: 'alice')) }
+  let(:receiver)       { Fabricate(:user) }
   let(:sender)         { Fabricate(:account, username: 'bob') }
   let(:foreign_status) { Fabricate(:status, account: sender, text: 'The body of the foreign status') }
   let(:own_status)     { Fabricate(:status, account: receiver.account, text: 'The body of the own status') }