remove use of std::rand in FollyTest

Summary: Fix linter complaint and make deterministic.

Reviewed By: yfeldblum

Differential Revision: D62766613

fbshipit-source-id: a1b66a7ee43dfd31dde42ba0a3374b8a7d70a84a

folly/memory/test/ArenaTest.cpp

@@ -283,7 +283,7 @@ TEST(Arena, Clear) {
   for (int i = 0; i < 10; ++i) {
     std::vector<size_t> sizes(1000);
     std::generate(sizes.begin(), sizes.end(), []() {
-      return std::rand() % blockSize * 2;
+      return Random::rand32(blockSize) * 2;
     });
 
     std::vector<void*> addresses;