add test_ping fix test_millis_mm
* Correct millis() drift, issue 3078 * Add 'test_millis_mm.ino' runtime benchmark * Eliminate 'punning' warning Add union 'acc' in millis() to eliminate 'punning' compiler warning * Correct minor typo * Eliminate 'punning' warning Add union 'acc' to eliminate 'punning' compiler warning in 'millis_test'_DEBUG() and 'millis_test()'