Add test data to mock out DMF API

[jasonaowen]

The DMF API in the cavetl repo is difficult to set up, mostly because it requires special access and permissions and so on. For the sake of demos, and to ease testing the PSM side of the DMF integration, add some test data and document how to use it.

We want this mock data in the PSM repository and not in the CAVETL repository for a few reasons:

• this is, like the code that consumes the DMF API, documentation of what the PSM expects
• these mocks are useful for testing and demonstrating the PSM
• these mocks are useful for writing integration tests
• the PSM's dependency on CAVETL does not include explicit version numbers or commit hashes, and if the DMF API implementation in the CAVETL repo changes, the PSM should be able to fall back to these mocks until it is updated to consume the new API (and these mocks should be updated to match)
• as a side effect of the previous point, it may be useful to be able to go back in time to previous versions of the PSM, and having these mocks eases that

(See also discussion on Zulip)

Thanks to @frankduncan for sharing the special SSNs and names returned by the Veris Test API.

Once this is merged, we can set up our CI to run this mock API before running the integration tests, and we can write integration tests around the PSM's DMF integration for #877!

Issue #759 Automatically check SSN against the Death Master File PR #877 Issue #759. Add DMF check for individuals Issue #1050 Add more integration tests

[jasonaowen]

Thanks, @frankduncan!