Revert "Avoid chdir error in bin/setup test on Ruby master"

This reverts commit a484bd8.

Author: yahonda

railties/test/application/bin_setup_test.rb

@@ -1,11 +1,10 @@
 # frozen_string_literal: true
 
 require "isolation/abstract_unit"
-require "chdir_helpers"
 
 module ApplicationTests
   class BinSetupTest < ActiveSupport::TestCase
-    include ActiveSupport::Testing::Isolation, ChdirHelpers
+    include ActiveSupport::Testing::Isolation
 
     setup :build_app
     teardown :teardown_app
@@ -29,7 +28,7 @@ def test_bin_setup
     end
 
     def test_bin_setup_output
-      chdir(app_path) do
+      Dir.chdir(app_path) do
         # SQLite3 seems to auto-create the database on first checkout.
         rails "db:system:change", "--to=postgresql"
         rails "db:drop", allow_failure: true