AHM ASAN test adjustment

Summary:
The AHA-insert-race test case (a) does a LOT of iterations, and
(b) allocated and deallocates a lot of memory on each iteration.

The long iterations, by themselves, take 8s, and should be reduced.
Further, ASAN errors on these tests, since it needs to hold on to memory
for a bit of time and ends up OOMing.

Test Plan:
fbconfig -r 'folly' '--platform-all=gcc-4.8.1-glibc-2.17-fb' '--sanitize=address'
fbmake runtests Ahm.atomic_hash_array_insert_race

Reviewed By: robbert@fb.com

Subscribers: sdwilsh, folly-diffs@

FB internal diff: D1767334

Tasks: 5941888

Signature: t1:1767334:1420612386:4d6f4067b30d1cb7d20bcfeb0f31fac1339ead20

folly/test/AtomicHashMapTest.cpp

@@ -606,7 +606,7 @@ void* atomicHashArrayInsertRaceThread(void* j) {
 }
 TEST(Ahm, atomic_hash_array_insert_race) {
   AHA* arr = atomicHashArrayInsertRaceArray.get();
-  int numIterations = 50000, FLAGS_numThreads = 4;
+  int numIterations = 5000, FLAGS_numThreads = 4;
   void* statuses[FLAGS_numThreads];
   for (int i = 0; i < numIterations; i++) {
     arr->clear();