Bump up memory capacity for ExchangeFuzzer

Summary:
facebookincubator#9186 increased the amount of memory the ExchangeFuzzer uses (it
changed the Values operator to copy its Vectors if it's multi-threaded to prevent concurrency issues).

Bumping up the ExchangeFuzzer's memory capacity allows it to continue to succeed.

Reviewed By: kgpai

Differential Revision: D56527484

fbshipit-source-id: 474a53031fe4cf79928b1face2dabc6a85efea26

velox/exec/tests/ExchangeFuzzer.cpp

@@ -526,7 +526,7 @@ int main(int argc, char** argv) {
   folly::Init init{&argc, &argv};
   memory::MemoryManagerOptions options;
   options.useMmapAllocator = true;
-  options.allocatorCapacity = 8UL << 30;
+  options.allocatorCapacity = 15UL << 30;
   options.useMmapArena = true;
   options.mmapArenaCapacityRatio = 1;
   memory::MemoryManager::initialize(options);