Fix TSAN timeout in MergeOperatorPinningTest.Randomized/x test

Summary:
[FB - Internal]
MergeOperatorPinningTest.Randomized/x tests are frequently failing with timeouts when run with tsan, as they are exceeding 10 minute limit for tests. The tests are in turn getting disabled due to frequent failures.
I halved the number of rounds to make the test complete sooner. This reduces the number of testing iterations a little, but it still is much better than totally letting the test be disabled.
Closes https://github.com/facebook/rocksdb/pull/3523

Differential Revision: D7031498

Pulled By: sagar0

fbshipit-source-id: 9a694f2176b235259920a42bf24bca5346f7cff1

db/db_merge_operator_test.cc

@@ -284,7 +284,7 @@ TEST_P(MergeOperatorPinningTest, Randomized) {
     Random rnd(301);
     std::map<std::string, std::string> true_data;
 
-    const int kTotalMerges = 10000;
+    const int kTotalMerges = 5000;
     // Every key gets ~10 operands
     const int kKeyRange = kTotalMerges / 10;
     const int kOperandSize = 20;