Move test data into a github release

[jcfr]

Look at SkullStripper extension for an example:

• https://github.com/Slicer/SkullStripper/blob/master/Testing/Cxx/CMakeLists.txt
• http://www.slicer.org/slicerWiki/index.php/Documentation/Nightly/Developers/Tutorials/UploadTestData

[fedorov]

Considering Midas is nearing its end of life, we should instead start hosting test data on github as "data releases".