Use GitHub instead of aws in remote dataset tests
Recently we switched from aws s3 to github to download dataset scripts.
However in the tests, the dummy data were still downloaded from s3.
So I changed that to download them from github instead, in the MockDownloadManager.

Moreover I noticed that `anli`'s dummy data were quite heavy (18MB compressed, i.e. the entire dataset) so I replaced them with dummy data with few examples.