Build: Run regression tests for lilypond-book (issue 2223).
Re-enable running the regtests in input/regression/lilypond-book.
Fix various typos, clarify what each input file tests for and
how the output should look, and fix the following problems
in input files in input/regression/lilypond-book:
collated-files.tely:
Was linking to the input files rather than the output files
generated by lilypond-book. The input files were thus not
tested individually as it should but as included files.
Fixed in the local GNUmakefile by an explicit rule for
collated-files.list which tracks the output files.
suffix-texi:
Failed, was not being preprocessed by lilypond-book but simply
copied to the output folder. Fixed in the local GNUmakefile by
adding a specific make rule for it.
tex-twocolumn:
Failed, image had full-page width.
Fixed in the input file by moving \twocolumn to the preamble.
Column width detection in the body of the tex document is not
yet implemented.
texinfo-language-detection:
Suspicious, had a weird @lydoctitle popping up.
Fixed in the input file by defining the @lydoctitle macro.