testing_diff_peaks: Fix test
testing_diff_peaks: Fix test

Better fix than that performed in a6c16838, and does not raise a compiler warning.

MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3