Improve test coverage for `backend/awssqs`

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What are we looking for?

We're looking to improve test coverage across the board. In this case, first start by checking the existing coverage by running make test/dev to get a sense of the existing coverage. Then take a look at existing tests to see how to cover the codebase. We'd like to improve coverage as we believe it gives folks higher confidence in the code that they are running.

There are already a number of tests for this, so it may be an easier one to pick up and start running with.

For an example PR that includes an update to tests check out https://github.com/batchcorp/plumber/pull/296/files