When run tests, where to see logs?

I’m developing a plugin, and followed the official development guide to write some test cases. My plugin code contains some log printed via kong.log.debug , kong.log.err, etc… In the kong_tests.conf, I have set log_level = debug. I also set prefix = servroot, I suppose to find log files under <kong path>/servroot/logs, but I cannot find that directory at all. Where logs go to?