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/update_account_service_spec.rb

@@ -5,9 +5,9 @@ RSpec.describe UpdateAccountService, type: :service do
 
   describe 'switching form locked to unlocked accounts' do
     let(:account) { Fabricate(:account, locked: true) }
-    let(:alice)   { Fabricate(:user, email: 'alice@example.com', account: Fabricate(:account, username: 'alice')).account }
-    let(:bob)     { Fabricate(:user, email: 'bob@example.com', account: Fabricate(:account, username: 'bob')).account }
-    let(:eve)     { Fabricate(:user, email: 'eve@example.com', account: Fabricate(:account, username: 'eve')).account }
+    let(:alice)   { Fabricate(:account) }
+    let(:bob)     { Fabricate(:account) }
+    let(:eve)     { Fabricate(:account) }
 
     before do
       bob.touch(:silenced_at)