Prevent regressions with requiring sprockets/railtie

Some applications or gems don't require sprockets/rails, instead they
load sprockets/railtie.

This change catches this issue when running tests.

test/test_helper.rb

@@ -2,7 +2,9 @@
 
 require 'action_view'
 require 'sprockets'
-require 'sprockets/rails'
+# Apps generated before Rails 7.0 without rails/all require sprockets/railtie, not sprockets/rails
+# Same with other reverse dependencies like sassc-rails
+require 'sprockets/railtie'
 require 'sprockets/rails/context'
 require 'sprockets/rails/helper'
 require 'rails/version'