Centralize testing/example data

[jhlegarreta]

What would you like to see added in this software?

The testing/example data is spread across two different repositories:
https://gin.g-node.org/nipreps-data/tests-nifreeze
https://github.com/nipreps-data/nifreeze-ismrm25-experiment2

We should keep the data at a single place so that all necessary testing/example data is hosted at a single place. This would ease automatically fetching the data as well.

Do you have any interest in helping implement the feature?

Yes

Additional information / screenshots

N/A.