Disable fsync in DBMergeOperatorTest to save test time (#7640)

Summary:
The test often times out in internal test infra.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7640

Test Plan: watch test to pass internally

Reviewed By: anand1976

Differential Revision: D24764928

Pulled By: cheng-chang

fbshipit-source-id: 587f2afc97f52909837943fd938a86ca94544b2c

db/db_merge_operator_test.cc

@@ -37,7 +37,7 @@ class TestReadCallback : public ReadCallback {
 class DBMergeOperatorTest : public DBTestBase {
  public:
   DBMergeOperatorTest()
-      : DBTestBase("/db_merge_operator_test", /*env_do_fsync=*/true) {}
+      : DBTestBase("/db_merge_operator_test", /*env_do_fsync=*/false) {}
 
   std::string GetWithReadCallback(SnapshotChecker* snapshot_checker,
                                   const Slice& key,