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/controllers/concerns/accountable_concern_spec.rb

@@ -12,14 +12,14 @@ RSpec.describe AccountableConcern do
     end
   end
 
-  let(:user)   { Fabricate(:user, account: Fabricate(:account)) }
-  let(:target) { Fabricate(:user, account: Fabricate(:account)) }
-  let(:hoge)   { Hoge.new(user.account) }
+  let(:user)   { Fabricate(:account) }
+  let(:target) { Fabricate(:account) }
+  let(:hoge)   { Hoge.new(user) }
 
   describe '#log_action' do
     it 'creates Admin::ActionLog' do
       expect do
-        hoge.log_action(:create, target.account)
+        hoge.log_action(:create, target)
       end.to change { Admin::ActionLog.count }.by(1)
     end
   end