fix guitarpro mtests

Aside from the "missing" key signatures and the resultant effect on the lid's of all subsequent elements, the diffs I saw were things where the tests are now correct but the original refs were not - like mmrests that used to be not generated because of the empty key sigs.  The hairpins and voltas in crescendo-and-diminuendo.gpx and volta.gpx seem correct, including after save / reload, so I'm assuming those too were "good" diffs.