Blacklist TransactionTest.GetWithoutSnapshot from valgrind_test (#5715)

Summary:
In valgrind_test, TransactionTest.GetWithoutSnapshot ran 2 hours and still didn't finish. Black list from valgrind_test to prevent timeout.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5715

Test Plan: run "make valgrind_test" and see whether the test is still generated.

Differential Revision: D16866009

fbshipit-source-id: 92c78049b0bc1c2b9a0dfc1b7c8a9206b36f02f0

utilities/transactions/transaction_test.cc

@@ -5488,6 +5488,7 @@ class ThreeBytewiseComparator : public Comparator {
   }
 };
 
+#ifndef ROCKSDB_VALGRIND_RUN
 TEST_P(TransactionTest, GetWithoutSnapshot) {
   WriteOptions write_options;
   std::atomic<bool> finish = {false};
@@ -5516,6 +5517,7 @@ TEST_P(TransactionTest, GetWithoutSnapshot) {
   commit_thread.join();
   read_thread.join();
 }
+#endif  // ROCKSDB_VALGRIND_RUN
 
 // Test that the transactional db can handle duplicate keys in the write batch
 TEST_P(TransactionTest, DuplicateKeys) {