Add testing of s3 upload functionality

[jlizen]

This came up in: #46

We don't really have any meaningful tests of s3 uploading functionality, or the aws functionality more general - just metadata parsing.

This forces manual validation when we make changes that touch on relevant code paths.

I see two main options:

• add unit tests using aws-smithy-mocks and keep doing manual e2e validation on significant changes as well
• add real e2e integration tests that we run using github CI against a real aws account, etc

I wouldn't say this is urgent, but will remain a paper cut until we automate something.

[arielb1]

I don't trust aws-smithy-mocks to actually detect any interesting error, and I don't like to have to maintain a real aws account just for testing s3.

[arielb1]

Now since #49, there are mocked tests, but I'm not sure I trust them for actually showing anything.