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

[scbedd]

Reference

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.