tests: store test uploads in temp dir.

Fixes parallel tests failing because uploads for different tests were
stored in the same directory and uploads got clobbered when cleaning up
this directory.

test/test_helper.rb

@@ -78,7 +78,7 @@ class ActiveSupport::TestCase
     mock_missed_search_service!
     WebMock.allow_net_connect!
 
-    storage_manager = StorageManager::Local.new(base_dir: "#{Rails.root}/public/data/test")
+    storage_manager = StorageManager::Local.new(base_dir: Dir.mktmpdir("uploads-test-storage-"))
     Danbooru.config.stubs(:storage_manager).returns(storage_manager)
     Danbooru.config.stubs(:backup_storage_manager).returns(StorageManager::Null.new)
   end