AtomicStore tests and test coverage

https://github.com/SpectrumXYZ/atomicstore/pull/4 was about a day and a half of unplanned work. Turned out to be a subtly in the Rust file system interface. AtomicStore is already used broadly throughout our code base and expected to be used even more. Bugs in this layer have the potential for addition difficult unplanned debugging.

We need to know
- How much code lacks adequate tests
- How much work is entailed in improving the tests

As a first step, `grcov` reports 71% of the code is exercised by tests. There are two tests that work together. The first generates and persists a bunch of simple numeric data. The second loads and checks this data.