Don't run if there was an exit with failure, i.e. exit(false).

This happens e.g. when there's a syntax error in a test file, and rake catches the error and calls `exit(false)` (in `Rake::Application#exit_because_of_exception`).  Without this fix, Minitest goes on to run all the tests that it had successfully loaded before the error, and finishes by saying there were no failures or errors.