Try to fix ExternalSSTFileTest.IngestNonExistingFile flakines (#4625 An error occurred )

Summary:
ExternalSSTFileTest.IngestNonExistingFile occasionally fail for number of SST files after manual compaction doesn't go down as expected. Although I don't find a reason how this can happen, adding an extra waiting to make sure obsolete file purging has finished before we check the files doesn't hurt.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4625

Differential Revision: D12910586

Pulled By: siying

fbshipit-source-id: 2a5ddec6908c99cf3bcc78431c6f93151c2cab59

db/external_sst_file_test.cc

@@ -1317,7 +1317,8 @@ TEST_F(ExternalSSTFileTest, IngestNonExistingFile) {
   ASSERT_OK(Flush());
 
   ASSERT_OK(db_->CompactRange(CompactRangeOptions(), nullptr, nullptr));
-
+  ASSERT_OK(dbfull()->TEST_WaitForCompact(true));
+  
   // After full compaction, there should be only 1 file.
   std::vector<std::string> files;
   env_->GetChildren(dbname_, &files);