Update test_box assertion for lazy-loaded error_highlight

error_highlight is no longer in $LOADED_FEATURES at boot time since
it is now loaded lazily via Exception#detailed_message hook.

[Feature #21951]

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: hsbt

test/ruby/test_box.rb

@@ -656,7 +656,7 @@ def test_prelude_gems_and_loaded_features
       assert_match EXPERIMENTAL_WARNING_LINE_PATTERNS[1], error[1]
 
       assert_includes output.grep(/^before:/).join("\n"), '/bundled_gems.rb'
-      assert_includes output.grep(/^before:/).join("\n"), '/error_highlight.rb'
+      refute_includes output.grep(/^before:/).join("\n"), '/error_highlight.rb'
       assert_includes output.grep(/^after:/).join("\n"), '/bundled_gems.rb'
       assert_includes output.grep(/^after:/).join("\n"), '/error_highlight.rb'
     end