[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.