input/test/ is no more

[Graham Percival]

We've finally finished processing all the files in input/test/.  I've 
removed it from input/GNUmakefile and Documentation/index.html.in

I have _not_ modified make/lilypond-vars.make, since I'm still building 
the docs via the "make web" method (ie I'm not building lilypond at 
all), and I don't want to break something without realizing it.  I think 
that it's a simple matter of deleting "-I $(input-dir)/test/" from that 
file, but I'd rather leave that to somebody else to do.

I also have _not_ delete the entire input/test/ directory; again, I 
think it's safer if somebody who actually builds lilypond does that.

Cheers,
- Graham