DA: Develop testing environment

## Summary

Since the DA is a complex process that must interpret simulation traces and communicate information back to a client, it deserves a comprehensive set of tests. This issue is to design and create a testing environment that will make constructing tests as easy as possible, while still being meaningful and powerful.

## Completion Criteria
* Have a testing environment where a mock client can initialize and talk to the DA server.
* The mock client will need to send various requests and verify the DA responds appropriately.

## Questions
Broadly speaking, we can see two ways of testing DA responses:
* Manually construct expected requests and responses.
* Record transcript between a real client and server, and replay in testing, to see if responses match what we expect.

Each has their advantage, so we may even want to implement both.