AppVeyor: More testing
AppVeyor: More testing

- Explicitly install python3 (seems to be absent as MRtrix3 configure script is not properly run; though on the Windows machine I have at hand, python -> python3 and python2 requires explicit installation...)

- When checking the status of the run_tests script, re-run the fc command. A previous run shows that testing failed, yet AppVeyor reported success; perhaps the subsequent "appveyor PushArtifact" call was resulting in a return code of zero, such that failure of the test script was not detected.
MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3