Is there a public instance of Splunk that can be used to test queries?
I Googled "public splunk test instance" and didn't see anything there.
No, there is no publicly available Splunk. However, Splunk is free to download and install on your laptop so there's always an instance available for you to test with.
You can try the Splunk cloud trial from https://www.splunk.com/page/sign_up/cloud_trial
But as Rich G said, it's free to install and run yourself