Rare bug fix

Summary:
By the birthday paradox, this will fail about once every 8,100
runs. Dropping to 100 with cut that to 1 in 860,000.

Test Plan: fbconfig -r folly && fbmake runtests

Reviewed By: sdoroshenko@fb.com

Subscribers: sdwilsh, njormrod

FB internal diff: D1581238

folly/test/RandomTest.cpp

@@ -51,7 +51,7 @@ TEST(Random, Simple) {
 }
 
 TEST(Random, MultiThreaded) {
-  const int n = 1024;
+  const int n = 100;
   std::vector<uint32_t> seeds(n);
   std::vector<std::thread> threads;
   for (int i = 0; i < n; ++i) {