Include Redis in Workhorse-in-test-suite integration

We started using Workhorse in feature tests, but no config.toml was provided. As a result, we missed a startup issue that could have been caught by testing the config loading.

A small subset of Workhorse functionality also relies on Workhorse and Rails sharing a Redis database. If we configure it here, we can enhance the runner API testing in rails to ensure it behaves as expected.

Closes #209245 (closed)