Gradle testkit can't add plugin to custom source set

When I try to execute this test:

https://gist.github.com/Frontrider/93c3a4fece9903698345f76fad57c85c

than it says that:

FAILURE: Build failed with an exception.

* Where:
Build file 'C:\Users\H87620143\AppData\Local\Temp\junit5825415720167959245\build.gradle' line: 1

* What went wrong:
A problem occurred evaluating root project 'junit5825415720167959245'.
> Plugin with id 'com.example.testplugin' not found.

I looked around a lot, (some of the things in my build.gradle are just trying out different things) and I can't get it to work.

The 'com.itsh.gradle.releasenotes.properties' file is generating under build/pluginDescriptors.