autotest: 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.

Tools/autotest/rover.py

@@ -5428,6 +5428,28 @@ def test_scripting_set_home_to_vehicle_location(self):
         self.context_pop()
         self.reboot_sitl()
 
+    def test_scripting_serial_loopback(self):
+        self.start_subtest("Scripting serial loopback test")
+
+        self.context_push()
+        self.context_collect('STATUSTEXT')
+        self.set_parameters({
+            "SCR_ENABLE": 1,
+            "SCR_SDEV_EN": 1,
+            "SCR_SDEV1_PROTO": 28,
+        })
+        self.install_test_script_context("serial_loopback.lua")
+        self.reboot_sitl()
+
+        for success_text in [
+                "driver -> device good",
+                "device -> driver good",
+        ]:
+            self.wait_statustext(success_text, check_context=True)
+
+        self.context_pop()
+        self.reboot_sitl()
+
     def Scripting(self):
         '''Scripting test'''
         self.test_scripting_set_home_to_vehicle_location()
@@ -5436,6 +5458,7 @@ def Scripting(self):
         self.test_scripting_simple_loop()
         self.test_scripting_internal_test()
         self.test_scripting_auxfunc()
+        self.test_scripting_serial_loopback()
 
     def test_mission_frame(self, frame, target_system=1, target_component=1):
         self.clear_mission(mavutil.mavlink.MAV_MISSION_TYPE_MISSION,