tests: fix cleanup of upload temp files.

Fix upload temp files sometimes not being cleaned up when tests exit.

test/test_helper.rb

@@ -50,10 +50,12 @@ class ActiveSupport::TestCase
   setup do
     Socket.stubs(:gethostname).returns("www.example.com")
 
-    @temp_dir = Dir.mktmpdir("danbooru-temp-")
+    @temp_dir = Dir.mktmpdir("danbooru-uploads-")
     storage_manager = StorageManager::Local.new(base_url: "https://www.example.com/data", base_dir: @temp_dir)
     Danbooru.config.stubs(:storage_manager).returns(storage_manager)
     Danbooru.config.stubs(:backup_storage_manager).returns(StorageManager::Null.new)
+
+    at_exit { FileUtils.rm_rf(@temp_dir) }
   end
 
   teardown do