Fix MigrateAttachmentsService spec when repo is already on disk

Running this after
spec/services/projects/hashed_storage_migration_service_spec.rb would fail
previously, because the repository already existed on disk. As this spec expects
the repository to not exist on disk prior to being created, we should clear out
the test repos before running.

spec/ee/spec/services/ee/projects/hashed_storage/migrate_attachments_service_spec.rb

@@ -14,6 +14,7 @@ describe Projects::HashedStorage::MigrateAttachmentsService do
 
     context 'on success' do
       before do
+        TestEnv.clean_test_path
         FileUtils.mkdir_p(FileUploader.dynamic_path_builder(old_attachments_path))
       end