Test against minio target storage

[cisaacstern]

In pangeo-forge-runner, @yuvipanda did an elegant job of setting up a minio instance to test recipes against s3-compatible storage. This is a nice way to catch oversights related to insufficiently generalized storage definitions. Probably a good thing to do here as well. See:

• https://github.com/pangeo-forge/pangeo-forge-runner/blob/35e77325d3453da9c71931ca31dbf3d05ac9b8be/tests/conftest.py#L33-L59
• https://github.com/pangeo-forge/pangeo-forge-runner/blob/35e77325d3453da9c71931ca31dbf3d05ac9b8be/tests/unit/test_bake.py#L56-L82