`livetest` output should be directly visible during in-progress test runs

[Reference](https://dev.azure.com/azure-sdk/internal/_build/results?buildId=6281870&view=logs&j=b33d6948-783e-5d93-1f76-4a84aaf50f45)

We've designed `dispatch_checks.py` to call the checks in a totally parallel way. Because of that, we don't see the output from a single test environment until it entirely completes. (to avoid mixed output)

This really sucks for livetests, where we are _often_ only invoking a single check + package AND the runtime can be super long. In the instances where we are invoking `dispatch_checks.py` for a single package, and a single check, run them serial with immediate output instead of collection.