Allow calling Timer.Reset from within the AfterFunc callback, fix tests

[gagern]

Allow calling Timer.Reset from within the AfterFunc callback. This is a more far-reaching request than https://github.com/benbjohnson/clock/pull/27 because it not only requires the mutex to be unlocked, but also the stop state to be fully registered by the time the function gets invoked.

Also fixes tests that have been broken for me.