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
http://www.iteachwithmoodle.com/2012/10/13/how-to-load-test-your-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.
https://docs.moodle.org/33/en/JMeter_test_plan_generator
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.