Feature request: add situation `test` to `eval-when`

[ceving]

I prefer having the unit tests right next to the definitions in the same source file. This makes it necessary to selectively enable them, because they should only be evaluated, when the code gets tested, but not when the code is running in production.

Right now I hijack the compile situation for my unit tests. But I think it would be cleaner to have a dedicated situation for this. Ideally this new situation can be enabled by a command line option.