Serial: Add basic tests for throughput and latency

java/libraries/serial/test/serial_throughput/arduino/serial_thoughput.ino

@@ -0,0 +1,9 @@
+void setup() {
+  Serial.begin(115200);
+}
+
+void loop() {
+  while (true) {
+    Serial.write('.');
+  }
+}