Tools: autotest: raise exception, don't return it

Spotted by @khancyr

Tools/autotest/common.py

@@ -545,7 +545,7 @@ def do_set_mode_via_command_long(self):
             if m.custom_mode == custom_mode:
                 return
             time.sleep(0.1)
-        return AutoTestTimeoutException()
+        raise AutoTestTimeoutException()
 
     def reach_heading_manual(self, heading):
         """Manually direct the vehicle to the target heading."""