How are vacuum optimized engines tested without disintegrating them? is an excellent question as it's attracted quite a number of informative and diverse answers.
@Uwe's answer informs us that there exist at least one or two low-pressure test chambers where engines can be fired for tens of seconds and the exhaust cleverly managed/diverted to maintain a pressure below 50 Torr for tens of seconds which is pretty amazing!
But the news discussed in the linked question says that
SpaceX recently test fired vacuum optimized raptor engines of starship... operated in atmosphere
@PcMan's answer tells us that this was indeed done at sea level, and says:
Apparently, the Raptor just bulls its way past the problem by virtue of very high chamber pressure (Meaning the nozzle is not so very overexpanded)
Question: How unusual is it to test vacuum engines at sea level? Is this yet another one of the many spaceflight-firsts it's pulled off? Or has it been done before?
This may be difficult to answer unambiguously; an engine meant to run in vacuum might be tested with a shorter nozzle at sea level, I don't think that should count. It should be a test that has the same configuration as designed for use in vacuum.