Fix backupable_db_test test cases that can't run by itself

Summary:
Several of backupable_db_test fails if running standalone, because of directory missing. Fix it by:
(1) garbage collector skips shared directory if it doesn't exit
(2) BackupableDBTest.Issue921Test to create the parent directory of the backup directory fist.

Test Plan: Run the tests individually and make sure they pass

Subscribers: leveldb, andrewkr, dhruba

Differential Revision: https://reviews.facebook.net/D56829

utilities/backupable/backupable_db.cc

@@ -1468,8 +1468,13 @@ Status BackupEngineImpl::GarbageCollect() {
     // delete obsolete shared files
     std::vector<std::string> shared_children;
     {
-      auto s = backup_env_->GetChildren(GetAbsolutePath(GetSharedFileRel()),
-                                        &shared_children);
+      auto shared_path = GetAbsolutePath(GetSharedFileRel());
+      auto s = backup_env_->FileExists(shared_path);
+      if (s.ok()) {
+        s = backup_env_->GetChildren(shared_path, &shared_children);
+      } else if (s.IsNotFound()) {
+        s = Status::OK();
+      }
       if (!s.ok()) {
         return s;
       }

utilities/backupable/backupable_db_test.cc

@@ -1341,8 +1341,8 @@ TEST_F(BackupableDBTest, ChangeManifestDuringBackupCreation) {
 TEST_F(BackupableDBTest, Issue921Test) {
   BackupEngine* backup_engine;
   backupable_options_->share_table_files = false;
+  env_->CreateDirIfMissing(backupable_options_->backup_dir);
   backupable_options_->backup_dir += "/new_dir";
-
   ASSERT_OK(BackupEngine::Open(env_, *backupable_options_, &backup_engine));
 
   delete backup_engine;