In this page of ReSharper options, you can adjust unit testing settings related to NUnit tests.
|Enable NUnit support||
By default, ReSharper 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.
Depending on these settings, ReSharper will use bundled NUnit test runners: v. 18.104.22.16850 and/or v. 3.0.5797.27553 correspondingly.
|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 NUnit2\addins subfolder of the ReSharper installation directory (which is shown for your convenience) and choose how to load them.