rocksdb: change db_test::MultiThreadedDBTest as value parameterized test.

Summary: This is a simple change to make db_test::MultiThreadedDBTest as value parameterized test. There is a value of creating a separate set of such tests later.

Test Plan:
```lang=bash
% make db_test
% ./make db_test
```

Also with the following command I can execute all db_test in 2:37.87 on my box
```
% ./db_test --gtest_list_tests | sed 's/\# GetParam.*//' | tr -d ' ' | env time parallel --gnu --eta --joblog=LOG -- 'TEST_TMPDIR=/dev/shm/rocksdb-{} ./db_test --gtest_filter="*{}"'
```

Reviewers: igor, rven, meyering, sdong

Reviewed By: meyering

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D35361

db/db_test.cc

@@ -8023,10 +8023,26 @@ static void MTThreadBody(void* arg) {
 
 }  // namespace
 
-TEST_F(DBTest, MultiThreaded) {
+class MultiThreadedDBTest : public DBTest,
+                            public ::testing::WithParamInterface<int> {
+ public:
+  virtual void SetUp() override { option_config_ = GetParam(); }
+
+  static std::vector<int> GenerateOptionConfigs() {
+    std::vector<int> optionConfigs;
+    for (int optionConfig = kDefault; optionConfig < kEnd; ++optionConfig) {
+      // skip as HashCuckooRep does not support snapshot
+      if (optionConfig != kHashCuckoo) {
+        optionConfigs.push_back(optionConfig);
+      }
+    }
+    return optionConfigs;
+  }
+};
+
+TEST_P(MultiThreadedDBTest, MultiThreaded) {
   anon::OptionsOverride options_override;
   options_override.skip_policy = kSkipNoSnapshot;
-  do {
   std::vector<std::string> cfs;
   for (int i = 1; i < kColumnFamilies; ++i) {
     cfs.push_back(ToString(i));
@@ -8059,10 +8075,12 @@ TEST_F(DBTest, MultiThreaded) {
       env_->SleepForMicroseconds(100000);
     }
   }
-    // skip as HashCuckooRep does not support snapshot
-  } while (ChangeOptions(kSkipHashCuckoo));
 }
 
+INSTANTIATE_TEST_CASE_P(
+    MultiThreaded, MultiThreadedDBTest,
+    ::testing::ValuesIn(MultiThreadedDBTest::GenerateOptionConfigs()));
+
 // Group commit test:
 namespace {