Deflaky ObsoleteFilesTest (#9049)

Summary:
WaitForFlushMemTable() may only wait for mem flush but not background flush
finishing. The the obsoleted file may not be purged yet.
https://github.com/facebook/rocksdb/blob/fcaa7ff6381fe6052b37a1d013b14960ea23ac17/db/db_impl/db_impl_compaction_flush.cc#L2200-L2203

Use WaitForCompact() instead to wait for background flush job.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/9049

Test Plan: `gtest-parallel ./obsolete_files_test --gtest_filter=ObsoleteFilesTest.DeleteObsoleteOptionsFile -r 1000`

Reviewed By: zhichao-cao

Differential Revision: D31737343

Pulled By: jay-zhuang

fbshipit-source-id: 82276ebeae7c7c75a733d3e1fd1c130d45e4761f

db/obsolete_files_test.cc

@@ -54,7 +54,9 @@ class ObsoleteFilesTest : public DBTestBase {
       AddKeys(numKeysPerFile, startKey);
       startKey += numKeysPerFile;
       ASSERT_OK(dbfull()->TEST_FlushMemTable());
-      ASSERT_OK(dbfull()->TEST_WaitForFlushMemTable());
+      ASSERT_OK(
+          dbfull()->TEST_WaitForCompact());  // wait for background flush (flush
+                                             // is also a kind of compaction).
     }
   }