MuseScore 01/07/2015: tests for rebuildMidiMapping()

I fixed small bug in checkMidiMapping() function: it decides whether to write MIDI mapping to a file or not.
Also I finally began to make tests for rebuildMidiMapping() and checkMidiMapping(). For now the following tests of *.mscx read/write are ready:

  • Read a file without MIDI mapping(MM) => write to a file without MM (with/without drum tracs or InstrumentChange elements).
  • Read a file with MM => write to a file with MM.
  • Read a file without MM, change it => write to a file with MM.
  • Read a file with MM, change the order of Parts => write to a file without MM.