autotest: erase logs after downloading

Tools/autotest/common.py

@@ -251,5 +251,8 @@ def log_download(mavproxy, mav, filename, timeout=60):
     mav.wait_heartbeat()
     mavproxy.send("log download latest %s\n" % filename)
     mavproxy.expect("Finished downloading", timeout=timeout)
+    mavproxy.send("log erase\n")
+    mav.wait_heartbeat()
+    mav.wait_heartbeat()
     return True