How to make paths in settings relative to the project dir

I'm trying to define a run/debug configuration to run my nose tests. If I click the [...] button to choose a Folder, it fills in the absolute path to my tests directory. That isn't good for working on projects cross-platform or for cloning branches. How do I make this path relative to the project root?

I tried

  $PROJECTDIR/tests

and

  ./tests

but they didn't work.

Thanks,

Matt