Add regression tests for lilypond-book (issue1556043)

[percival . music . ca]

The code looks ok, but I couldn't check it due to the docbook issue
(below).


http://codereview.appspot.com/1556043/diff/1/45
File make/lilypond-book-rules.make (right):

http://codereview.appspot.com/1556043/diff/1/45#newcode48
make/lilypond-book-rules.make:48: cd $(outdir) && dblatex $(notdir $<)
docbook is not currently a build requirement, and I'm very reluctant to
add it as such.  Could this be made optional?

also, it should probably be added as a configure check, use $(DBLATEX)
instead of a normal shell call, etc etc.  Actually, could this be
removed entirely from the first patch, then worked on as a separate
issue?

http://codereview.appspot.com/1556043/show