Try to fix DBSSTTest.RateLimitedDelete flakiness (#4840)

Summary:
DBSSTTest.RateLimitedDelete is flakey. The root cause is not completely identified, but
the compaction waiting in the test doesn't strictly wait for compaction cleaning to finish, which
may cause test flakiness. Fix it first and see whether the failures still happen.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4840

Differential Revision: D13567273

Pulled By: siying

fbshipit-source-id: 6fce38b912aff92a925231e7aa9bb0fef892761a

db/db_sst_test.cc

@@ -385,6 +385,7 @@ TEST_F(DBSSTTest, RateLimitedDelete) {
 
   // Compaction will move the 4 files in L0 to trash and create 1 L1 file
   ASSERT_OK(db_->CompactRange(CompactRangeOptions(), nullptr, nullptr));
+  ASSERT_OK(dbfull()->TEST_WaitForCompact(true));
   ASSERT_EQ("0,1", FilesPerLevel(0));
 
   uint64_t delete_start_time = env_->NowMicros();