How to include test classes into shadowJar?

I am using shadow Gradle plugin to build JAR, containing all referenced jars inside.

In my build.gradle I have only

apply plugin: "com.github.johnrengelman.shadow"

and

jar {
    manifest {
        attributes 'Main-Class': 'MYCLASS'
    }

}

related to that. I don't know, how it knows, what to build, but it works.

Now, is it possible, to include test classes too?