NAWS China Lake [0] is home to the EA-18G [1] test fleet. They carry the ALQ-99[2] jammer which can disrupt GPS quite effectively.

[0] https://en.wikipedia.org/wiki/Naval_Air_Weapons_Station_Chin...

[1] https://en.wikipedia.org/wiki/Boeing_EA-18G_Growler

[2] https://en.wikipedia.org/wiki/AN/ALQ-99

You'd think they could test it in a less populated area.

Maybe population was a part of it. There are so many things they could be doing/testing we can't really make any assumptions other than that some government organization is doing something.