autotest: fix race condition in mission-end-behaviour test

Tools/autotest/rover.py

@@ -5643,16 +5643,23 @@ Brakes have negligible effect (with=%0.2fm without=%0.2fm delta=%0.2fm)
                     if self.get_sim_time_cached() - tstart > 15:
                         self.progress("Got POSITION_TARGET_GLOBAL_INT, all good !")
                         break
-            self.change_mode("GUIDED")
 
             self.start_subtest("Test End Mission Behavior ACRO")
             self.set_parameter("MIS_DONE_BEHAVE", 2)
-            self.change_mode("AUTO")
+            # race conditions here to do with get_sim_time()
+            # swallowing heartbeats means we have to be a little
+            # circuitous when testing here:
+            self.change_mode("GUIDED")
+            self.send_cmd_do_set_mode('AUTO')
             self.wait_mode("ACRO")
 
             self.start_subtest("Test End Mission Behavior MANUAL")
             self.set_parameter("MIS_DONE_BEHAVE", 3)
-            self.change_mode("AUTO")
+            # race conditions here to do with get_sim_time()
+            # swallowing heartbeats means we have to be a little
+            # circuitous when testing here:
+            self.change_mode("GUIDED")
+            self.send_cmd_do_set_mode("AUTO")
             self.wait_mode("MANUAL")
             self.disarm_vehicle()
         except Exception as e: