Reserve the vectors in ConcurrentHashMapTest (#998)

Summary:
Even though this is a test, we are still reallocating a vector (resizing until re-sized/reallocated to 32), which in theory calls for allocation 5 times. But we can save this by just reserving the vector pre-hand (we know the size).
Pull Request resolved: https://github.com/facebook/folly/pull/998

Reviewed By: magedm

Differential Revision: D13834292

Pulled By: yfeldblum

fbshipit-source-id: 6b0d16ef0200184d10ba292a0b1d8d3afdadf56d

folly/concurrency/test/ConcurrentHashMapTest.cpp

@@ -342,6 +342,7 @@ TEST(ConcurrentHashMap, UpdateStressTest) {
   }
   std::vector<std::thread> threads;
   unsigned int num_threads = 32;
+  threads.reserve(num_threads);
   for (uint32_t t = 0; t < num_threads; t++) {
     threads.push_back(lib::thread([&, t]() {
       int offset = (iters * t / num_threads);
@@ -394,6 +395,7 @@ TEST(ConcurrentHashMap, EraseStressTest) {
   }
   std::vector<std::thread> threads;
   unsigned int num_threads = 32;
+  threads.reserve(num_threads);
   for (uint32_t t = 0; t < num_threads; t++) {
     threads.push_back(lib::thread([&, t]() {
       int offset = (iters * t / num_threads);