Add MinIO fixture to get full test coverage across S3 unit tests

[jhiemstrawisc]

Right now, our S3 unit tests point to public AWS buckets, but this comes with several limitations like the inability to test writes and the inability to test authorized reads.

Instead of hosting special CI infrastructure that exposes buckets for testing, we should figure out how to spin up an instance of MinIO inside the unit test framework that creates these buckets locally. Then we should develop test coverage for the following

• public read, 1MB file, 1GB file, both path/virtual hosted buckets
• authorized read, 1MB file, 1GB file, both path/virtual hosted buckets
• authorized write, 1MB file, 1GB file (requires resolution of S3 backend can't PUT files that don't fit in a single buffer from XRootD #39 first), both path/virtual hosted buckets
• ls of a public prefix (i.e. a bucket with no access/secret keys), both path/virtual hosted buckets
• ls of a private prefix (i.e. a bucket that requires access/secret keys), both path/virtual hosted buckets