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

[delcypher]

Summary

[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.

[kcc]

LGTM

compiler-rt/trunk/lib/fuzzer/tests/FuzzerUnittest.cpp

for (int i = 0; i < NumIter; i++) {
     if (NewSize == 7 && !memcmp(CH2, T, 7)) FoundMask |= 1 << 2;
     if (NewSize == 7 && !memcmp(CH3, T, 7)) FoundMask |= 1 << 3;
     if (NewSize == 7 && !memcmp(CH4, T, 7)) FoundMask |= 1 << 4;
   }
   EXPECT_EQ(FoundMask, 31);
 }
 
 TEST(FuzzerMutate, ShuffleBytes1) {
-  TestShuffleBytes(&MutationDispatcher::Mutate_ShuffleBytes, 1 << 16);
+  TestShuffleBytes(&MutationDispatcher::Mutate_ShuffleBytes, 1 << 17);
 }
 TEST(FuzzerMutate, ShuffleBytes2) {
   TestShuffleBytes(&MutationDispatcher::Mutate, 1 << 20);
 }
 
 void TestCopyPart(Mutator M, int NumIter) {
   std::unique_ptr<ExternalFunctions> t(new ExternalFunctions());
   fuzzer::EF = t.get();