[LibFuzzer] Fix ``FuzzerMutate.ShuffleBytes2`` unit test on OSX.

The ``FuzzerMutate.ShuffleBytes2`` unit test was failing on
OSX due to the implementation of ``std::random_shuffle()``
being different between libcxx and libstdc++.

@kcc has decided (see http://reviews.llvm.org/D21218) it is acceptable
for there to be different mutation behavior on different platforms so
this commit just adjusts the test to perform the minimum number of
iterations (that is a power of 2) to see all the mutations the unit test
is looking for.

Differential Revision: http://reviews.llvm.org/D21359

llvm-svn: 272743