Hopefully fix flakiness in rednet test

src/test/server-files/computers/computer/13/startup.lua

@@ -2,11 +2,11 @@
 
 rednet.open("top")
 
-rednet.send(14, "Test msg")
-
 local id, msg
 repeat
-    id, msg = rednet.receive()
+    rednet.send(14, "Test msg") -- Keep sending, as other computer may not have started yet.
+
+    id, msg = rednet.receive(nil, 1)
     print(id, msg)
 until id == 14