Add test for /backup

[alxndrsn]

Closes #1305

What has been done to verify that this works as intended?

Ran tests.

Why is this the best possible solution? Were any other approaches considered?

There are probably better solutions, like:

1. checking the backup tar.gz files can be reconstructed
2. checking the backup actually restores

1 may stray into the realms of testing pg_dump itself.

How does this change affect users? Describe intentional changes to behavior and behavior that could have accidentally been affected by code changes. In other words, what are the regression risks?

This change should not affect users.

Does this change require updates to the API documentation? If so, please update docs/api.yaml as part of this PR.

No.

Before submitting this PR, please make sure you have:

• run make test and confirmed all checks still pass OR confirm CircleCI build passes
• verified that any code from external sources are properly credited in comments or that everything is internally sourced

[matthew-white]

I think this could probably use a rebase now that #1310 is merged.

[alxndrsn]

I think this could probably use a rebase now that #1310 is merged.

I will update prior to requesting review.