[LibFuzzer] Try to unbreak the `FuzzerMutate.ShuffleBytes1` unit test.

This test is failing on my Linux box. Just increasing the number of
iterations works around this. The divergence is likely due to
our reliance on `std::shuffle()` which is not guaranteed to have
the same behaviour across platforms.

This is a strong argument for us to implement our own shuffle
function to avoid divergence in behaviour across platforms.

Differential Revision: https://reviews.llvm.org/D45767

llvm-svn: 330390