Build: Run regression tests for lilypond-book (issue 2223).

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.