Signal (SIGINFO/SIGUSR1?) to print current state

[silentbicycle]

The theft test runner could behave like dd and other programs when sent SIGINFO:

Set a flag that makes theft print progress stats and the shrunken instance(s), so far, before the next shrinking attempt.

This should be possible to disable in the config, in case that the code under test uses SIGINFO/SIGUSR1, but could be a good default behavior.

[silentbicycle]

The behavior here may not be clear in the presence of multi-core searching/shrinking, since there may be multiple trials being investigated concurrently.