Is LEGO testing the market with the Minifigure Series?
That sounds like a very valid question, but I think the answer is slightly more complex.

First, as you note, there's an obvious link in terms of moulds reuse, which means LEGO can offload the cost of these new parts to the minifig series (which aren't exactly on the cheap side), so that regular sets introduced later can be cheaper as a result. They do that too with licensed sets, by the way; for example, the Harley Quinn minifig financed the cost of the jester headgear.

Then, as you ask, do they actually test the market?

In a way, this would seem to make sense: if, say, spartan minifigs sell well, LEGO might be tempted to introduce spartan sets. Yet I think there are two problems with that.

The first is that they need to follow which minifig is more sought after, which they can't study from their own data since they sell "random" minifigs and the buyer can't pick the ones they want. Of course, they can check the aftermarket for that, bricklink prices and so forth, but is that really representative of what fans want, I don't know.

The second point is that sets design happens quite a long time before sets reach the shops. A designer told me in july 2012 that all 2013 sets were already designed, so you can count on 1 year and half between design and shops. Moreover, designing sets isn't the first step, and picking themes come before that. So if LEGO feels that a particular minifig sells well and decide to make a theme based on it, it'll be two years before that theme appears. Which isn't to say they won't, but I think they're a bit more preemptive than that.

So, in my opinion, LEGO might decide to do whole themes which have similarities with collectible minifigs, but I doubt they take the time to test the market - they just use the minifigs to offset the cost of new parts in upcoming sets. So, will we see cavemen sets? Spartan sets? Noone but LEGO can say at this point, but it's a possibility. It seems there will be some western sets in 2013, and we've indeed seen indian minifigs in the first series.