Fix the flaky cursor persist test (#10250)

Summary:
The 'PersistRoundRobinCompactCursor' unit test in `db_compaction_test` may occasionally fail due to the inconsistent LSM state. The issue is fixed by adding `Flush()` and `WaitForFlushMemTable()` to produce a more predictable and stable LSM state.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/10250

Test Plan: 'PersistRoundRobinCompactCursor' unit test in `db_compaction_test`

Reviewed By: jay-zhuang, riversand963

Differential Revision: D37426091

Pulled By: littlepig2013

fbshipit-source-id: 56fbaab0384c380c1f279a16dc8732b139c9f611

db/db_compaction_test.cc

@@ -5225,7 +5225,8 @@ INSTANTIATE_TEST_CASE_P(
 TEST_F(DBCompactionTest, PersistRoundRobinCompactCursor) {
   Options options = CurrentOptions();
   options.write_buffer_size = 16 * 1024;
-  options.max_bytes_for_level_base = 64 * 1024;
+  options.max_bytes_for_level_base = 128 * 1024;
+  options.target_file_size_base = 64 * 1024;
   options.level0_file_num_compaction_trigger = 4;
   options.compaction_pri = CompactionPri::kRoundRobin;
   options.max_bytes_for_level_multiplier = 4;
@@ -5241,6 +5242,7 @@ TEST_F(DBCompactionTest, PersistRoundRobinCompactCursor) {
     for (int j = 0; j < 16; j++) {
       ASSERT_OK(Put(rnd.RandomString(24), rnd.RandomString(1000)));
     }
+    ASSERT_OK(Flush());
   }
 
   ASSERT_OK(dbfull()->TEST_WaitForCompact());