Remove a race condition between lsdir and rm (#4440)

Summary:
In DBCompactionTestWithParam::ManualLevelCompactionOutputPathId, there is
a race condition between `DBTestBase::GetSstFileCount` and
`DBImpl::PurgeObsoleteFiles`. The following graph explains why.

```
Timeline  db_compact_test_t              bg_flush_t         bg_compact_t
    |  [initiate bg flush and
    |      start waiting]
    |                                     flush
    |                                     DeleteObsoleteFiles
    |  [waken up by bg_flush_t which
    |   signaled in DeleteObsoleteFiles]
    |
    |  [initiate compaction and
    |   start waiting]
    |
    |                                                         [compact,
    |                                                          set manual.done to true]
    |                                   [signal at the end of
    |                                    BackgroundCallFlush]
    |
    |  [waken up by bg_flush_t
    |   which signaled before
    |   returning from
    |   BackgroundCallFlush]
    |
    |  Check manual.done is true
    |
    |  GetSstFileCount    <-- race condition -->           PurgeObsoleteFiles
    V
```
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4440

Differential Revision: D10122628

Pulled By: riversand963

fbshipit-source-id: 3ede73c39fee6ad804dc6ac1ed84759c7e63977f

db/db_compaction_test.cc

@@ -2477,6 +2477,7 @@ TEST_P(DBCompactionTestWithParam, ManualLevelCompactionOutputPathId) {
 
     // Compaction range overlaps files
     Compact(1, "p1", "p9", 1);
+    ASSERT_OK(dbfull()->TEST_WaitForCompact());
     ASSERT_EQ("0,1", FilesPerLevel(1));
     ASSERT_EQ(1, GetSstFileCount(options.db_paths[1].path));
     ASSERT_EQ(0, GetSstFileCount(options.db_paths[0].path));
@@ -2492,6 +2493,7 @@ TEST_P(DBCompactionTestWithParam, ManualLevelCompactionOutputPathId) {
 
     // Compact just the new range
     Compact(1, "b", "f", 1);
+    ASSERT_OK(dbfull()->TEST_WaitForCompact());
     ASSERT_EQ("0,2", FilesPerLevel(1));
     ASSERT_EQ(2, GetSstFileCount(options.db_paths[1].path));
     ASSERT_EQ(0, GetSstFileCount(options.db_paths[0].path));
@@ -2508,6 +2510,7 @@ TEST_P(DBCompactionTestWithParam, ManualLevelCompactionOutputPathId) {
     compact_options.target_path_id = 1;
     compact_options.exclusive_manual_compaction = exclusive_manual_compaction_;
     db_->CompactRange(compact_options, handles_[1], nullptr, nullptr);
+    ASSERT_OK(dbfull()->TEST_WaitForCompact());
 
     ASSERT_EQ("0,1", FilesPerLevel(1));
     ASSERT_EQ(1, GetSstFileCount(options.db_paths[1].path));