fix mtest failures by updating all mscx files

by doing a

for f in $(find mtest -name '*.mscx')
do
   echo $f
   <path-to>mscore -t -o $f %f 2>/dev/null
done

Worked for all but mscore/guitarpro/tremolobar.gpx-ref.mscx, which
crashes on load, see #30836