Restore autoloading test for STI

This commit restores the test deleted in

    fc7c029467

Active Record should not test things about autoloading, this would
belong to the railties test suite probably. However, there, it feels
a bit too distant from here.

Imperfect, but on a second thought I believe this trade-off is better.

activerecord/test/cases/inheritance_test.rb

@@ -1,6 +1,7 @@
 # frozen_string_literal: true
 
 require "active_support/inflector"
+require "zeitwerk"
 require "cases/helper"
 require "models/author"
 require "models/company"
@@ -373,6 +374,19 @@ class InheritanceTest < ActiveRecord::TestCase
     end
   end
 
+  def test_new_with_autoload_paths
+    path = File.expand_path("../models/autoloadable", __dir__)
+    Zeitwerk.with_loader do |loader|
+      loader.push_dir(path)
+      loader.setup
+
+      firm = Company.new(type: "ExtraFirm")
+      assert_equal ExtraFirm, firm.class
+    ensure
+      loader.unload
+    end
+  end
+
   def test_inheritance_condition
     assert_equal 12, Company.count
     assert_equal 3, Firm.count