There is no command to insert data for testing. However, there are lots of ways to set up to test a search command:
Since search commands don't change any data, why not just search on the production system? Cut the time range back to "last 15 minutes" or "last 4 hours" to minimize the search time.
If you want to upload some test data, why not install a copy of free Splunk (or the trial license) on your laptop or desktop? Once you have a test instance set up, you can upload some real data and then use it to test your searches, etc. Or use some test data...
If you don't want to install a test server somewhere, you could create a test index and upload some data to it. Then just add index=test at the beginning of you search.