Add integration tests for detecting and responding to a Challenge

[geoknee]

          > This LGTM, but wanting for a follow-up with tests that consume the same.

I added a test here add estUpdateWithChallengeRegisteredEvent but as you say it doesn't cover that much code at present. Be nice to get an integration test going with a chain and engine and then we could assert on some notification perhaps?

Originally posted by @geoknee in https://github.com/statechannels/go-nitro/issues/1593#issuecomment-1699402729

[lalexgap]

https://github.com/statechannels/go-nitro/pull/1593#issuecomment-1699645809

[geoknee]

#1618