[Rainmaker]

Massive betatests are not likely to happen in the future because they were there only to test capability of sa-mp to handle that much players.

Beta testing of sa-mp is run in small and close group of players, who are chosen and invited by developer. See list of betatesters on following link.
http://forum.sa-mp.com/showgroups.php

Private tests of single gamemodes are pretty much random. Visit specific webpages of those gamemodes.