Feature Request: Write tests for the s3-pull extension

[crayolakat]

Problem
The s3-pull contact loader extension currently doesn't have any tests.

Solution
Create a Jest test for the s3-pull contact loader extension.

Context
The AWS.S3 function will need to be mocked: https://github.com/MoveOnOrg/Spoke/blob/2e8c479ef7e4071bc903bdf882334488b50f2910/src/extensions/contact-loaders/s3-pull/index.js#L92

This may entail needing to refactor some code, although there's probably another way to do it

[crayolakat]

@lastminutediorama suggested to consider adding localstack to our docker-compose dev containers for this because it's pretty handy for local s3 testing