The speed of light test. Cut the source. What happens?

If light travels at 299,792 km sec, like ‘they’ say it does, a laser light should carry on travelling, after the source is cut. But. If when the source is cut, all light that was radiating off of it, disappears too, that light is absolute. There will be no definitive reading, but the ‘absolute or not’ question, will be answered. Slam Dunk.