I will make some quick distinctions for you. Some times people (not saying you) tend to jumble MMA and the individual disciplines that make it up into one category.
In Brazilian Jiu Jitsu for example, AAS usage is greatly frowned upon. Mass and power are contrary to several of its key tenants. Foremost of those tenants being technique and leverage as the primary catalyst to a better game (victory). They supercede strength/size/explosiveness in all known (respected) adaptations of JJ.
As a reference point, check out the BJJ forums. Conversely, observe the stark contrast on the MMA forums. Although already implied, MMA is wrought with 'users'. AAS and rec drugs alike.
I'm a BJJ blue belt (staring at purple) who trains at a respected Gracie gym that also trains MMA, Muay Thai guys etc. Not to make a sweeping, all enveloping generalization but the personalties are remarkably different.
*EDIT* No, very few MMA contests are tested. Actually, none that I know of outside the upper echelons of competition. I know 40-50 MMA fighters. I've never heard the word 'test'. No pun intended.