Add explicit support for detecting bad latencies and ratios

@r0mant @jeremyschlatter please review
What do you think about replacing `map[float64]bool` here with `[]float64`, and then creating the map from the slice in the `for` loop below? The advantage I see is less visual noise in looking at the test cases that would make it easier to understand what the intended result was in each case.

I thought about this in your last PR, too, but didn't feel strongly enough about it to mention it.