Fix test_box assertion for lazy-loaded error_highlight

error_highlight is now lazy-loaded, so it should not be in
$LOADED_FEATURES at boot time.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: hsbt

test/ruby/test_box.rb

@@ -656,7 +656,8 @@ 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'
+      # error_highlight is lazy-loaded, so not in $LOADED_FEATURES at boot
+      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