check Rails configuration before modifying system test

Fixes #559

With this change the scaffold generator no longer assumes the Rails
project has system tests enabled.

Author: rschultheis

lib/generators/test_unit/scaffold/scaffold_generator.rb

@@ -4,7 +4,7 @@ module TestUnit # :nodoc:
   module Generators # :nodoc:
     class ScaffoldGenerator < Base # :nodoc:
       def fix_system_test
-        if turbo_defined?
+        if system_tests_enabled? && turbo_defined?
           gsub_file File.join("test/system", class_path, "#{file_name.pluralize}_test.rb"),
                     /(click_on.*Destroy this.*)$/,
                     "accept_confirm { \\1 }"
@@ -16,6 +16,10 @@ def fix_system_test
       def turbo_defined?
         defined?(Turbo)
       end
+
+      def system_tests_enabled?
+        defined?(Rails.configuration.generators) && Rails.configuration.generators.system_tests
+      end
     end
   end
 end