Fix intermittently failing S3 test

[ktuite]

Hopefully closes #1208

Did a bunch of sleuthing and saw that uploadPending was trying to upload 0 pending blobs. I overlooked aBlobExistsWith at first because I thought it was checking an assertion or something, but when I finally revisited what it was really doing (inserting a blob), I noticed this way in which it could be contributing to failing tests.

This would not have been possible without the previous PR (#1213) to clean up around the tests, so thanks!

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

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

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?

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

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]

Makes sense to me. 👍

[alxndrsn]

🤦 unawaited promises, probably the cause of 50% of test flake