offer something that will make the time investment worthwhile. for example every battle you play you get fuel points wins gain more. after so many battles you get a token that will allow you to rent a ship from the normal server arsenal. these ships would function like normal with cost for repair etc.... but to run them you need fuel which can only be earned by doing battles in the test server and if they want people playing a certain tier they could offer more points for that tier etc... so for example 20 battles earns you the first token then 60 for the next then 100. and a loss gives you 10 fuel win 15. you need 100 fuel to do a norm battle 200 for clan battles. so people willing to put time in the test server get something fun or worthwhile for it. and if the arsenal ships are too much then offer some other ones.