Grails - What is mixin and its significance in Grails Testing?

I am learning Grails Testing and in the first paragraph it says that

Grails 2.0.x and above deprecates these test harnesses in favour of **mixins** that can be applied to a range of different kinds of tests (JUnit 3, JUnit 4, Spock etc.) without subclassing

I researched about it found that it is being used for special multiple inheritance in other language. Can somebody explain what is mixin and its significance in Grails Testing?