autotest: do timesync roundtrip to ensure fresh system time

validating stream rates by checking we have received SYSTEM_TIME is no good if we receive data out of a tcp queue somewhere.

Tools/autotest/common.py

@@ -2059,6 +2059,7 @@ class AutoTest(ABC):
 
     def set_streamrate(self, streamrate, timeout=20, stream=mavutil.mavlink.MAV_DATA_STREAM_ALL):
         '''set MAV_DATA_STREAM_ALL; timeout is wallclock time'''
+        self.do_timesync_roundtrip(timeout_in_wallclock=True)
         tstart = time.time()
         while True:
             if time.time() - tstart > timeout: