Attempt to deflake DBSSTTest.DBWithSFMForBlobFilesAtomicFlush (#9241)

Summary:
When using the SST file manager, the actual deletion of DB files
potentially occurs in the background. The patch adds another call
to `SstFileManagerImpl::WaitForEmptyTrash` to the test case
`DBSSTTest.DBWithSFMForBlobFilesAtomicFlush` to ensure the deletions
are performed before the test checks the number of deleted files.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/9241

Test Plan:
```
gtest-parallel --repeat=1000 ./db_sst_test --gtest_filter=DBSSTTest.DBWithSFMForBlobFilesAtomicFlush
```

Reviewed By: akankshamahajan15

Differential Revision: D32811427

Pulled By: ltamasi

fbshipit-source-id: 7f2ad649a22bd2d7900e5f132372034093cfcf47

db/db_sst_test.cc

@@ -1664,14 +1664,20 @@ TEST_F(DBSSTTest, DBWithSFMForBlobFilesAtomicFlush) {
   ASSERT_OK(db_->CompactRange(CompactRangeOptions(), begin, end));
 
   ASSERT_EQ(files_added, 1);
-  ASSERT_EQ(files_deleted, 1);
   ASSERT_EQ(files_scheduled_to_delete, 1);
 
+  sfm->WaitForEmptyTrash();
+
+  ASSERT_EQ(files_deleted, 1);
+
   Close();
   ASSERT_OK(DestroyDB(dbname_, options));
+
+  ASSERT_EQ(files_scheduled_to_delete, 4);
+
   sfm->WaitForEmptyTrash();
+
   ASSERT_EQ(files_deleted, 4);
-  ASSERT_EQ(files_scheduled_to_delete, 4);
 
   ROCKSDB_NAMESPACE::SyncPoint::GetInstance()->DisableProcessing();
   ROCKSDB_NAMESPACE::SyncPoint::GetInstance()->ClearAllCallBacks();