How to disable NUnit Adapter for given project?

I am pretty sure that all test adapters that you have installed run for all assemblies every time you build, NUnit is just a bit more verbose about it which is very annoying.

Tests won't be discovered though if the Test Explorer window is not open. Unfortunately, closing the window isn't enough, you need to close it, then restart Visual Studio.

Update: Based on the comments, also make sure that Keep Test Execution Engine Running is not checked.

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The following is on my system running Visual Studio 2015 Update 2;

Test Explorer Window Closed, Rebuild All;

Notice, no test discovery or Test in the dropdown. You can also see in the Test | Test Settings that the test engine is not running because the menu options are disabled.

As soon as I open Test Explorer however;

Even if I close the window, tests continue to be discovered until I restart Visual Studio.