move CANCELED flag into a global lazy_static so it can be reset and re-used on multiple tests
move Gaggle-shutdown handling to new SHUTDOWN_GAGGLE flag (also a global lazy_static) so that a Gaggle test shutting down at the same time as a non-Gaggle test is running won't cause both to be shut down
test coverage with --run-time configured
test coverage without --run-time configured (ie, run forever until canceled)
test coverage with --iterations configured
test coverage with --test-plan including canceling while the test is Increasing, Maintaining, and Decreasing
adds dev-dependency on nix in order to access kill for sending SIGINT
--run-timeconfigured--run-timeconfigured (ie, run forever until canceled)--iterationsconfigured--test-planincluding canceling while the test is Increasing, Maintaining, and Decreasingnixin order to accesskillfor sendingSIGINT