db_test: clean up sync points in test cleaning up

Summary: In some db_test tests sync points are not cleared which will cause unexpected results in the next tests. Clean them up in test cleaning up.

Test Plan:
Run the same tests that used to fail:

build using USE_CLANG=1 and run
./db_test --gtest_filter="DBTest.CompressLevelCompaction:*DBTestUniversalCompactionParallel*"

Reviewers: rven, yhchiang, igor

Reviewed By: igor

Subscribers: leveldb, dhruba

Differential Revision: https://reviews.facebook.net/D36429

db/db_test.cc

@@ -481,6 +481,9 @@ class DBTest : public testing::Test {
   }
 
   ~DBTest() {
+    rocksdb::SyncPoint::GetInstance()->DisableProcessing();
+    rocksdb::SyncPoint::GetInstance()->LoadDependency({});
+    rocksdb::SyncPoint::GetInstance()->ClearAllCallBacks();
     Close();
     Options options;
     options.db_paths.emplace_back(dbname_, 0);