Having said that, there's still something I don't get.

If a group is a possible configuration to load packages, then it seems that the "reasonable" possible configurations would be:

1 - Core

2 - Tests + Core (explicitly stated)

3 - Default (Tests + Core, implicit when not specified)

but NOT

Tests (without the Core)

Because you wouldn't want to load the tests of a package without the Core.

So, why would you define the Tests as a possible configuration for a Version?