Aggregate most recent runs

This method significantly reduces noise when applied in combination of
Mann-Whitney U test. It replaces standard deviation estimation feature, as it
clearly performs better.

It isn't enabled by default.


git-svn-id: https://llvm.org/svn/llvm-project/lnt/trunk@209204 91177308-0d34-0410-b5e6-96231b3b80d8