Testing that no grob is created

[Dan Eble]

I have a regression test that should fail if a BarLine grob is created.  It is 
sufficient to override BarLine.glyph-name to make it obvious in the visual diff.

Is there a simpler, more direct, or more robust way to test this?

I suppose I could add an engraver that warns about each BarLine it 
acknowledges, and enable the warning-as-error option.  Any other ideas?
— 
Dan