Regtests: add missing descriptions (half fix 1206) (issue2103043)

[percival . music . ca]

Reviewers: ,

Message:
The song-* stuff will be done later (without review, since nobody
particularly cares about the festival stuff, and those regtest names are
pretty explanatory anyway).


Description:
Regtests: add missing descriptions (half fix 1206)

This adds descriptions to all regtests other than festival song-*.

Please review this at http://codereview.appspot.com/2103043/

Affected files:
  M input/regression/beam-concave-chord.ly
  M input/regression/figured-bass-slashed-numbers.ly
  M input/regression/flags-straight-stockhausen-boulez.ly
  M input/regression/les-nereides.ly
  M input/regression/mozart-hrn-3.ly
  M input/regression/predefined-fretboards.ly
  M input/regression/stem-tremolo-forced-dir.ly