Unit testing blob creation, stashing and processing using BlobManager.stashed

[dannimad]

BlobManager.stashed added a new way to unit test blob manager, but only focused around stashing expired/ non expired blobs. The pattern is also useful for unit testing any other public methods in the blob manager so the goal is to add more unit testing to eventually have one that can replicate AB#4630 scenario.

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✅ Successfully linked to Azure Boards work item(s):

• Bug 4630

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Latest commit: 027759df40ce7fc38de8fcecbf78bbda91b38423

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This PR has been automatically marked as stale because it has had no activity for 60 days. It will be closed if no further activity occurs within 8 days of this comment. Thank you for your contributions to Fluid Framework!