Fix assert_template_result tests not picking up Liquid::Environment.default.error_mode

The `rake test` command gave us the impression that we were running all the tests
on all the error modes, that was false.

test/test_helper.rb

@@ -34,7 +34,7 @@ module Minitest
 
     def assert_template_result(
       expected, template, assigns = {},
-      message: nil, partials: nil, error_mode: nil, render_errors: false,
+      message: nil, partials: nil, error_mode: Liquid::Environment.default.error_mode, render_errors: false,
       template_factory: nil
     )
       file_system = StubFileSystem.new(partials || {})