autotest: drain mav before attempting to run a command

We're losing an arbitrary amount of data while getting the ACK, so it's
fine to lose whatever is in the buffer.

Tools/autotest/common.py

@@ -2952,6 +2952,7 @@ class AutoTest(ABC):
                 target_compid=None,
                 timeout=10,
                 quiet=False):
+        self.drain_mav_unparsed()
         self.get_sim_time() # required for timeout in run_cmd_get_ack to work
         self.send_cmd(command,
                       p1,