Ability to test expressions in edit modals

**Is your feature request related to a problem? Please describe.**

I'm always frustrated when I have to save an expression and open a different modal to test it

**Describe the solution you'd like**

I'd like to be able to test expressions (e.g. expression policy, scope mapping, ...) in the same modal as the editor.

This would make it easier & quicker to write & test policies, and also safer as there would be no need to save a potentially broken policy before testing it.