message: add a getBufSize API to the TestPacketBatchWriter class
  Summary: utitlity API to verify test packet writing output
diff: @@ -371,6 +371,10 @@ class TestPacketBatchWriter : public IOBufBatchWriter {
  folly::AsyncUDPSocket& /*unused*/,
  const folly::SocketAddress& /*unused*/) override;
  + size_t getBufSize() const {
  + return bufSize_;
  + }
  +
  private:
  int maxBufs_{0};
  int bufNum_{0};
  