Add note that test are run in same process group as ts

[thinkerbot]

The result of which is that if you kill the process group within a test then you kill ts itself. This is hard to avoid because (AFAIK) setting the process group id requires system-specific things like setsid, or a programming language like ruby with Process.setsid, or maybe job control in bash. Moreover, you might want to keep it this way so that signals sent to the ts group will go to all ts tests... hard to say, good to make a note about.

ps -j -p $$
test_process_groups () {
  ps -j -p $$
  bash -c 'ps -j -p $$'
  false
}