Issue 3968: Improve output of scheme-engraver.ly regtest.

Issue 3968: Improve output of scheme-engraver.ly regtest.

I was reading the scheme-engraver.ly regtest, trying to understand scheme engravers, but I spent most of my time trying to understand the convoluted scheme code and terminal output.  This just cleans it up a little bit so future users don't have to waste time like I did.  By the way, is there a reason this is a regtest, and not in "Extending"?

Thanks
- Mark

http://code.google.com/p/lilypond/issues/detail?id=3968

OLD OUTPUT:
(initialize #<Mom -infinity>
)(start-trans #<Mom 0>
)(process-music #<Mom 0>  
)(process-acknowledged #<Mom 0>  
)(saw head:  #<Grob NoteHead >  coming from  #<Translator Note_heads_engraver >)(process-acknowledged #<Mom 0>  

NEW OUTPUT:
#<Mom -infinity>: (initialize)
#<Mom 0>        : (start-translation-timestep)
#<Mom 0>        : (process-music)
#<Mom 0>        : (process-acknowledged)
#<Mom 0>        : saw #<Grob NoteHead > coming from #<Translator Note_heads_engraver >
#<Mom 0>        : (process-acknowledged)