tools: add arc4random_uniform() bias test

This test program executed arc4random_uniform() repeatedly and
analyzes the distribution of return values, showing how similar
the parameters of the observed outcome are to the expected
parameters of an equidistribution.

This cannot be a unit test as it takes quite a while to run and
lots of memory (~3 GB) to execute.

Reviewed by: cem
Approved by: emaste
Differential Revision: https://reviews.freebsd.org/D47659