test we're uploading trailing checksums

[sbrudenell]

I'm evaluating our threat model. I realize we should calculate and send checksums for uploaded objects. I'd previously dismissed this as redundant with the integrity guarantee of TLS. But it may also provide defense against long-term bit rot in the cloud, assuming the provider doesn't do any other common method like block level checksums, and does use the customer-provided checksum to detect long-term bit rot, which isn't documented anywhere.

Per https://docs.aws.amazon.com/AmazonS3/latest/userguide/checking-object-integrity.html#trailing-checksums, this is easy to do in boto3.