Spock Tests In MPP

I have a 1.3.61 MPP project according to the docs with JVM and JS targets and everything is working fine so far. I would like to test the JVM side with Spock Tests written in Groovy but I have no clue how to go about this. Can someone please give me a pointer?

Btw, the tests already exist so moving to Spek is not really what I want to do.