Fix a flaky test with test sync point (#5310)

Summary:
If DB is opened with `avoid_unnecessary_blocking_io` being true, then `~ColumnFamilyHandleImpl` enqueues a purge request and schedules a background thread to perform the deletion. Without test sync point, whether the SST file is purged or not at a later point in time is not deterministic. If the SST does not exist, it will cause an assertion failure.

How to reproduce:
```
$git checkout 6492430eaf
$make -j20 deletefile_test
$gtest-parallel --repeat 1000 --worker 16 ./deletefile_test --gtest_filter=DeleteFileTest.BackgroundPurgeCFDropTest
```
The test may fail a few times.
With changes made in this PR, repeat the above commands, and the test should not fail.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5310

Differential Revision: D15361136

Pulled By: riversand963

fbshipit-source-id: c4308d5f8da83472c893bf7f8ceed347fbfa850f

db/deletefile_test.cc

@@ -305,6 +305,7 @@ TEST_F(DeleteFileTest, BackgroundPurgeCFDropTest) {
                    &sleeping_task_after, Env::Priority::HIGH);
     // If background purge is enabled, the file should still be there.
     CheckFileTypeCounts(dbname_, 0, bg_purge ? 1 : 0, 1);
+    TEST_SYNC_POINT("DeleteFileTest::BackgroundPurgeCFDropTest:1");
 
     // Execute background purges.
     sleeping_task_after.WakeUp();
@@ -318,6 +319,13 @@ TEST_F(DeleteFileTest, BackgroundPurgeCFDropTest) {
     do_test(false);
   }
 
+  SyncPoint::GetInstance()->DisableProcessing();
+  SyncPoint::GetInstance()->ClearAllCallBacks();
+  SyncPoint::GetInstance()->LoadDependency(
+      {{"DeleteFileTest::BackgroundPurgeCFDropTest:1",
+        "DBImpl::BGWorkPurge:start"}});
+  SyncPoint::GetInstance()->EnableProcessing();
+
   options_.avoid_unnecessary_blocking_io = true;
   ASSERT_OK(ReopenDB(false));
   {
@@ -326,6 +334,7 @@ TEST_F(DeleteFileTest, BackgroundPurgeCFDropTest) {
   }
 
   CloseDB();
+  SyncPoint::GetInstance()->DisableProcessing();
 }
 
 // This test is to reproduce a bug that read invalid ReadOption in iterator