AP_Scripting: add serial loopback test script

Tests that data can flow both ways with one end using protocol 28
(Scripting) and the other using the serial device feature.

libraries/AP_Scripting/tests/serial_loopback.lua

@@ -0,0 +1,36 @@
+local ser_driver = serial:find_serial(0)
+local ser_device = serial:get_device_port(1)
+
+if ser_driver == nil or ser_device == nil then
+  error("bad config")
+end
+
+ser_driver:begin(115200) -- baud rate does not matter
+
+function test_driver_to_device()
+  local msg_send = "hello device"
+  local num_sent = 0
+  for ci = 1,#msg_send do
+    num_sent = num_sent + ser_driver:write(msg_send:byte(ci, ci)):toint()
+  end
+  local msg_recv = ser_device:readstring(#msg_send)
+  if msg_send == msg_recv and num_sent == #msg_send then
+    gcs:send_text(6, "driver -> device good")
+  end
+end
+
+function test_device_to_driver()
+  local msg_send = "hello driver"
+  local num_sent = ser_device:writestring(msg_send)
+  local msg_recv = ser_driver:readstring(#msg_send)
+  if msg_send == msg_recv and num_sent == #msg_send then
+    gcs:send_text(6, "device -> driver good")
+  end
+end
+
+function update()
+  test_driver_to_device()
+  test_device_to_driver()
+end
+
+return update()