Fix a order-dependant test failure in railties

Reproduce with `bin/test --seed 27653`

Closes rails#53368

Co-authored-by: zzak <[email protected]>

Author: Earlopain

railties/test/application/configuration/custom_test.rb

@@ -5,6 +5,8 @@
 module ApplicationTests
   module ConfigurationTests
     class CustomTest < ActiveSupport::TestCase
+      include ActiveSupport::Testing::Isolation
+
       def setup
         build_app
         FileUtils.rm_rf("#{app_path}/config/environments")