Move test data to a release attachment

[gdevenyi]

wiki.mouseimaging.ca is regularly having issues, having the data somewhere might be a good idea

[bcdarwin]

See https://github.com/Mouse-Imaging-Centre/RMINC-test-data.

PR #312, when merged, will modify the tests to pull from there.

[gdevenyi]

Cool, missed that. Thanks.