autotest: drain mav before attempting disarm

If the queue of messages for us to parse is very long then we can time
out before managing to parse them all and find the disarm!

Tools/autotest/common.py

@@ -1088,6 +1088,7 @@ class AutoTest(ABC):
     def disarm_vehicle(self, timeout=60, force=False):
         """Disarm vehicle with mavlink disarm message."""
         self.progress("Disarm motors with MAVLink cmd")
+        self.drain_mav_unparsed()
         p2 = 0
         if force:
             p2 = 21196 # magic force disarm value