This test requires a USB device implementing the USB CDC-ACM protocol configured to loop back TX to RX. For example, this Arduino sketch could be used:

void setup() {
  Serial.begin(115200);
  Serial.setTimeout(0);
  while (!Serial) {
    ;
  }
}

void loop() {
  if (Serial.available()) {
    char buf[1024]; // Greater than the endpoint packet size.
    int count = Serial.readBytes(buf, sizeof buf);
    Serial.write(buf, count);
  }
}