How to set a different system property for each jmv gradle spawning during parallel testing?

Question

Gradle allows me to run multiple jvms for testing as follows:

test { maxParallelForks = 10 } 

Some tests for the application that I have require a fake ftp server that needs a port. This is pretty easy to do with one jvm:

 test { systemProperty 'ftpPort', 10000 } 

However, when running in parallel, I will need to start 10 fake ftp servers. How to add a custom system property for every jvm created by gradle?

Something like:

 test { maxParallelForks 10 customizeForks { index -> systemProperty 'ftpPort', 10000 + index } }