Signal progress of the test to avoid timeout

Summary: Closes https://github.com/facebook/rocksdb/pull/2824

Differential Revision: D5756457

Pulled By: maysamyabandeh

fbshipit-source-id: dff53e945d8ac4ffe6775a2176424fd1a27fc189

utilities/transactions/write_prepared_transaction_test.cc

@@ -362,6 +362,8 @@ TEST_P(WritePreparedTransactionTest, SnapshotConcurrentAccessTest) {
   for (size_t old_size = 1;
        old_size <= WritePreparedTxnDB::DEF_SNAPSHOT_CACHE_SIZE + 2;
        old_size++) {
+    printf("."); // To signal progress
+    fflush(stdout);
     const std::vector<SequenceNumber> old_snapshots(
         snapshots.begin(), snapshots.begin() + old_size);