Convert timer_* to a utility class and convert tests to phpunit

This patch moves the timer_* functions to a utility class so it can be unit tested outside of drupal, and converts the existing test to phpunit.

The original test was waiting way too long (4 seconds) and only asserting greater than, so the accuracy of the timers wasn't well tested. The new assertions test both greater and less than, and reduces the sleep time to 5ms.