Configure IntelliJ correctly so sourcesets are marked as test

[DanielThomas]

https://youtrack.jetbrains.com/issue/IDEA-151925#comment=27-2355076

[rpalcolea]

@DanielThomas I don't think we have this issue right? it seems that our source sets are marked as test when created. At least the ones that use test as parent.

[heowc]

I added idea as a plugin.

plugins {
    ...
    id 'idea'
    ...
}