How can we use splunk in software testing like in finding bugs of use of any automation tool with splunk?
This question is much too broad to be answered in any useful way.
Here is the broad answer that fits with your broad question:
"Collect the log files or outputs from the automation tools; put them into Splunk and look for errors."
Or you can consider looking for details like 'passwords captured in plain text and stored in logs', SSN or Credit Card Numbers in your logs.