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/models/public_feed_spec.rb

@@ -31,7 +31,6 @@ RSpec.describe PublicFeed, type: :model do
     end
 
     it 'filters out silenced accounts' do
-      account = Fabricate(:account)
       silenced_account = Fabricate(:account, silenced: true)
       status = Fabricate(:status, account: account)
       silenced_status = Fabricate(:status, account: silenced_account)
@@ -176,8 +175,7 @@ RSpec.describe PublicFeed, type: :model do
 
       context 'with language preferences' do
         it 'excludes statuses in languages not allowed by the account user' do
-          user = Fabricate(:user, chosen_languages: [:en, :es])
-          @account.update(user: user)
+          @account.user.update(chosen_languages: [:en, :es])
           en_status = Fabricate(:status, language: 'en')
           es_status = Fabricate(:status, language: 'es')
           fr_status = Fabricate(:status, language: 'fr')
@@ -188,8 +186,7 @@ RSpec.describe PublicFeed, type: :model do
         end
 
         it 'includes all languages when user does not have a setting' do
-          user = Fabricate(:user, chosen_languages: nil)
-          @account.update(user: user)
+          @account.user.update(chosen_languages: nil)
 
           en_status = Fabricate(:status, language: 'en')
           es_status = Fabricate(:status, language: 'es')
@@ -199,7 +196,8 @@ RSpec.describe PublicFeed, type: :model do
         end
 
         it 'includes all languages when account does not have a user' do
-          expect(@account.user).to be_nil
+          @account.update(user: nil)
+
           en_status = Fabricate(:status, language: 'en')
           es_status = Fabricate(:status, language: 'es')