That's one test, but I didn't think it was the 'Miller' test. The way I remember Miller is that the sawed-off shotgun was disallowed because it wasn't useful as a military weapon. (Though indeed it was in WW1, some shenanigans about who was allowed to testify happened in Miller). The Miller Case was about military usefulness, not indiscriminate results.
"Testimony" per se doesn't occur at the Supreme Court, which is not a trial court. Briefs are filed, the various lawyers present oral arguments, (but this can include "friends of the court", I think, in any event they can file briefs) and the Justices decide on the basis of those arguments, and the record of the case to that point. In the Miller case, the defense lawyer had moved to throw out the case because the law in question violated the Second Amendment, and thus was no law at all, and the trial judge agreed. Bang, Case dismissed. The government appealed *directly* to the Supreme Court, not through the Court of Appeals. No one appeared to argue against the government, including the NRA. Still the court, wanting to find for the government, ruled that the lower court erred in determining that keeping and bearing a short barreled shotgun was protected by the second amendment, *without* taking testimony or seeing evidence that such a weapon was suitable for use by a member of a Well Regulated militia, not that that the person protected needed to be such a member at the time of keeping and bearing. But since Miller was dead and his co defendant Layton choose to plead guilty in exchange for probation, there were no "further proceedings" at which such testimony could be heard, or evidence examined.