Only run tests that match two or more groups

[ghost]

I asked a question a while ago about being able to run tests that only match multiple groups. I think it got misinterpreted so I have reworded it. However I think the answer is a no.

Therefore I think it would be a nice feature to implement an ability to only run tests where the groups specified in the command are mutually inclusive to the groups labeled on a test. (Mutually inclusive may not be correct terminology - my maths is rusty)

[sebastianbergmann]

I would consider a pull request that implements this but won't implement it myself.

[carcinocron]

I would love for --group unit+controllers to be implemented and behavior intuitively different from --group unit,controllers.