[llvm-exegesis] Fix a warning in r332221

comparison of integers of different signs: 'const unsigned long' and 'const int' [-Werror,-Wsign-compare]

unittests/tools/llvm-exegesis/BenchmarkResultTest.cpp:60:5: note: in instantiation of function template specialization 'testing::internal::EqHelper<false>::Compare<unsigned long, int>' requested here
    ASSERT_EQ(FromDiskVector.size(), 1);

llvm-svn: 332230