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There's not really a formula, because it depends on where and what that mass is, plus other factors. To take one extreme, if it's all a beer gut, then it would (simplistically! - see below) be a simple linear relationship: lose 10% of your weight, and you'd accelerate 10% faster. But at the other extreme, if the excess weight is all in your leg muscles, losing weight would decrease your speed.
But all that's almost irrelevant, because much depends on the type of muscle you have (the ratio of fast-twitch to slow-twitch fibers), how well they're trained, the exact shape of your body (biomechanics), the efficiency of your respiration (for anything but short sprints), and much more. If the running test is still 2 miles, it's going to be a test of your aerobic capacity more than anything, so as cbaber says, you have to aim your training at increasing that. Of course reducing non-muscle weight is going to help, and - the good part - the training will help reduce weight.
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