add the option to randomly test one target out of a set

[ben-jones]

Right now, our platform test against all targets every time it is run. This is problematic because a) we want to be mindful of bandwidth constraints and so we need to either probabilistically select targets from a set or use a very small set of targets and b) we will be less fingerprintable if we don't test our entire URL list every time.

[ben-jones]

@gsathya, @rpanah, this is basically a util/measurement primitive and from #59, it seems like we are pushing measurement primitive creation to after the alpha release. Therefore, I am dropping this primitive from the alpha.