do not modify random sequence at startup

Thanks to Ned Batchelder for pointing out a problem with importing SymPy
wherein the random sequence is modified.

http://nedbatchelder.com/blog/201302/hunting_a_random_bug.html

Since function arguments are executed at definition time, the function
modified in runtests was advancing the random sequence as SymPy was
imported.
Merge pull request #1785 from smichr/random

do not modify random sequence at startup