Make MillServerMain and TestRunner kill themselves when a new process starts using the folder

[lihaoyi]

Currently, both of these processes have a tendency to runaway, leaving a bunch of test runners or mill servers spinning in the background eating up CPU and battery on your laptop.

While robustifying the Ctrl-C killing logic would be nice, another thing we could do is make them periodically check a PID file to see if they are still the only ones running in a folder, and if not then kill themselves. This is currently what .runBackground does, which seems to work well enough, and it would mean that a runaway process would only survive until the next mill ... command before dying.

[lihaoyi]

Fixed in #3370