There are a few types of open-source load testing tools, with JMeter being the most popular one.
Here is a nice tutorial on how to load test Moodle server using JMeter
There is a built in tool in Moodle called the "JMeter test plan generator" which allows you to run performance tests over a Moodle.
Consider also to simulate network performance and security with a tool like Ixia's IxChariot. It can simulate real applications to predict device, system, and network performance under realistic load conditions.