What are you trying to stress test? The communications or the printer overall speed? We do a huge amount of stress testing at Zebra for both, but it would be fairly easy to write an app to test. Simply create a handful of ZPL formats, then send them over the communication channel that you are concerned about over and over. Add in maybe some ~HS requests to get print status to really verify that it's still handling the load. At some point, the buffers will fill up and the printer should handle it in standard ways with the communication protocols. In our app testing we will often take a roll of labels, rip off about 4 feet, string it through the back of the printer and tape the ends together to not waste labels.
Some Zebra printers are designed to go into high speed production line environments and have to perform with the standards of quality that customers expect. Often if the printer goes down, so does the line. Even printers that are not designed for those environments get thoroughly tested to meet stress factors and share the same OS to handle those situations.