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/batched_remove_status_service_spec.rb

@@ -5,7 +5,7 @@ RSpec.describe BatchedRemoveStatusService, type: :service do
 
   let!(:alice)  { Fabricate(:account) }
   let!(:bob)    { Fabricate(:account, username: 'bob', domain: 'example.com') }
-  let!(:jeff)   { Fabricate(:user).account }
+  let!(:jeff)   { Fabricate(:account) }
   let!(:hank)   { Fabricate(:account, username: 'hank', protocol: :activitypub, domain: 'example.com', inbox_url: 'http://example.com/inbox') }
 
   let(:status1) { PostStatusService.new.call(alice, text: 'Hello @bob@example.com') }