could you tell us what test you need - receive (so the wireless card is just listening and not transmitting) or transmit (so the wireless is constantly transmitting frames)?
Is this possible with mikrotik? I have needed something simple like this in the past. What I have done was to set up two systems with 2 computers running iperf and a spectrum analyzer. It was a pain due to the number of parts. A simpler setup would be great.