Add acceptance tests for /runs endpoints

[mattwelch]

#10187 left the tests for robot-server's /runs endpoints in a state of partial overlap. The files that test that resources persist across reboots are also, partially, testing that those resources are correct. This ticket is to split things up to give each test file a clearer job.

Acceptance criteria

For the /runs endpoints:

• Tavern tests should cover the bulk of behavior. In particular, they assert that the response bodies are semantically correct.
• The non-Tavern persistence tests should only cover persistence. They should rely on other tests to make sure the server is persisting the correct things.