In this page of JetBrains Rider options, you can adjust unit testing settings related to NUnit tests.
Enable NUnit support
By default, JetBrains Rider adds action indicators allowing you to execute NUnit tests right from the editor and automatically discovers NUnit tests in your solution. Clear check boxes corresponding to specific NUnit versions to disable this behavior.
Custom NUnit installation
Use specified custom NUnit
If you want to use another NUnit version, tick this check box and enter the path to the local NUnit installation into the corresponding text box.
Load NUnit Addins
If support of NUnit 2x is selected above and you need to use NUnit addins for this bundled version, put the addins in the lib\ReSharperHost\NUnit2\addins subfolder of the JetBrains Rider installation directory (which is shown for your convenience) and choose how to load them.