NUnit

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. 2.6.4 and/or v. 3.6.1 correspondingly.

If you want to use another NUnit version, tick this checkbox and enter the path to the local NUnit installation into the corresponding field

Notes:

• ReSharper 2019.2 supports NUnit 3.10.1 and later.

• If there are any NUnit addins in this custom installation, they will be loaded too.

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.

Starting from NUnit 3, current directory is not set by the NUnit framework (see this answer in the NUnit tracker). ReSharper sets the current directory to the temp folder by default, but you can change it.