But I stumbled over your first sentence: “One of the predictions of special relativity is that the speed of light in a vacuum is a universal constant.”

I’d always thought that the speed-of-light-is-a-constant was a postulate (along with the laws-are-the-same-in-inertial-frames) .. not a prediction.

I’ve seen derivations of special relativity where only the relativity principle is assumed. In these alternative derivations, a maximum signal speed pops out as a parameter (which may be infinite, reproducing Galilean relativity). Then you derive that something propagating at the speed hasinteresting properties (like constant speed in all inertial frames). So the experimental test is to see if you can find something with those special properties, and you can .. it’s light.