folly: ubsan: reduce vector size to avoid UBSAN timeout

Summary:
Based on diff where this was introduced in {D360195} it seems like

```
  // This value should we multiple of word size.
  static size_t const kHeapifyCapacitySize = sizeof(
    typename std::aligned_storage<
      sizeof(InternalSizeType),
      alignof(value_type)
    >::type);
  // Threshold to control capacity heapifying.
  static size_t const kHeapifyCapacityThreshold =
    100 * kHeapifyCapacitySize;
```

So anything above 100*sizeof(SizeType) should do.

Reviewed By: philippv

Differential Revision: D2871422

fb-gh-sync-id: a69e47286c53887ac05e89dab565b9d609e183a0

folly/test/small_vector_test.cpp

@@ -663,7 +663,7 @@ TEST(small_vector, Capacity) {
 
   // Test capacity heapifying logic
   folly::small_vector<unsigned char, 1> vec3;
-  const size_t hc_size = 1000000;
+  const size_t hc_size = 100000;
   for (size_t i = 0; i < hc_size; ++i) {
     auto v = (unsigned char)i;
     vec3.push_back(v);