Test datasets are not valid BIDS datasets

[arokem]

Might be a minor issue for us, but while I was debugging #870 I ran into issues validating our synthetic datasets (see #870 (comment)). If we want these datasets to be valid BIDS data (not sure why that might be necessary for us...), we'll need to use something like this gist https://gist.github.com/arokem/7505a75031924960fc9c18a88589900e

[arokem]

One annoying aspect of this is that apparently nifti files have to be above a certain size for the bids validator to consider them valid neuroimaging data, which is why I had to expand the synthetic data to (100, 100, 100) instead of the (10, 10, 10) that we currently use.