#### Problem
Having a PR that is self-contained with a big picture while having it testable
for TieredStorage::write_accounts is challenging, as it requires all the write-side
code, read-side code, and test code.
#### Summary of Changes
This PR solves part of the problem by having one dedicated PR for tests.
Specifically, it includes test utils that allow us to generate test accounts,
invoke write_accounts, and verify the written tiered-storage instance.
With this PR, it will be easier to write future PRs.
#### Test Plan
A new set of unit-tests and test utils is included in this PR.
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