That sort of "control" wouldn't necessarily be conclusive; Bing could be using click data weighed by a prior calculation of how trustworthy the clicked-on site is (e.g. an analogue of PageRank score), whereupon clicks on Google would rank much higher than clicks on some newly-created fake search engine.

I guess my point is, unless you have deep knowledge into the secret workings of a search engine with hundreds of inputs, simple "tests" to figure out the nature of just one of those inputs are bound to end in multiple plausible explanations for your results, unless you rig a huge fraction of the WWW. And Google has better things to do with their time.