Testing Gradle Plugin implementation which uses evaluation

I am currently developing plugins for our internal projects which allow central configuration.
For this I write tests, which work really great with the ProjectBuilder.

But now I come across the following problem:

Gradle Version: 6.2.2
Kotlin 1.61

I’m adding new Source Sets which are added via afterEvaluate{} to avoid forcing the order of the plugins.
Now I can’t find a way to test this via JUnit.

In the internet I find project.evaluate() which leads to compilation errors.
An afterEvaluate{} block makes no sense, because the assertions are not executed.

How can I force the project to be evaluated before I execute the assertions?