+ Refactored inner loop of Reporter#report to #to_s. Callable for status updates.
+ Moved io.sync restore to Reporter#run_and_report.
[git-p4: depot-paths = "//src/minitest/dev/": change = 8571]
+ Added Runnable.with_info_handler and Runnable.on_signal.
+ Restored MT4's mid-run report (^t). (tenderlove).

[git-p4: depot-paths = "//src/minitest/dev/": change = 8573]