To put it in a modern context, imagine if a group that a lot of people think of as a "cult" snuck in and poked Obama in the middle of wartime to get a sample of his blood for some reason that the general public would think is weird. (Presumably only the Jedi and small number of others truly understand the significance of midichlorians). If this "cult" were caught doing this. Immediately, that cult would be looked at with incredible suspicion. So, this scenario would play directly into Palpatine's hands and allow him to depict the Jedi as traitors. Plus, the midicholorian count would only prove that he's strong in the Force, not that he's evil or a Sith. Plus, they'd have to figure that it'd be pretty hard to get a sample of his midichlorians. He's the chancellor. He's presumably pretty heavily guarded. Poking somebody is a pretty obvious thing. (And I don't know of anyone getting a midichlorian count from saliva.) Plus, if he is Force sensitive, he's probably going to sense that you're going to poke him, and that would mean that you most likely wouldn't be able to. So, the fact that he is Force sensitive would make it all but impossible for you to test that he is Force sensitive. Overall, I think the pros of testing him are so strongly outweighed by the cons that trying to test him would not only be a bad idea but also a foolish move by the Jedi.