Enable logging for rspec on Logstash plugin

I'm making some changes to the logstash-input-salesforce plugin and would like to be able to see the logging generated by @logger.debug statements when running rspec tests.
Currently, when rspec starts, it prints Sending Logstash logs to null which is now configured via log4j2.properties , but it's not obvious to me how to change that and get it to send Logstash logs to a file, for example.

Any ideas? Do I create a log4j2.properties somewhere? If so, where?

I'm very new to both Ruby and Logstash plugin development so this might be very obvious to a more experienced dev.

Thanks,
Frans