According to the
wiki page it was in 2008, and they found no significant "stealth capability" worth noting. And remember that the test sample was just a wooden skin - no engines or wheels and none of the massive steel-tube spaceframe structure of the actual aeroplane.
There are some silly claims that one of the 229s achieved ~730mph, but these people had clearly been smoking something they shouldn't have. Even if they had the power (which they didn't, with less than 4,000lbs of installed thrust) to fly at that speed they'd be well beyond Mcrit, and the short control moment to the wing-tip elevons couldn't have provided anything like the elevator power needed to oppose the the transonic trim change even if they worked at all at that speed. This was what destroyed the similarly-configured DH108s.
PDR