MSVC's uniform random number generator is not quite as uniform as
others, requiring a slightly wider threshold on the histogram test.
After inspecting histograms for several runs, there's no obvious
bias -- just some bins end up having slightly more less elements
(often > 2% but less than 2.5%).
This provide several advantages:
- more flexibility in designing unit tests
- unit tests can be glued to speed up compilation
- unit tests are compiled with same predefined macros, which is a requirement for zapcc
