Issue description

Currently we run tests directly after a package has been build. Unfortunately, we need to disable tests regularly to avoid circular dependencies caused by test dependencies.

Running tests in a separate derivation would:

• allow us to test all packages because circular dependencies are no longer a issue;
• reduce build time of the actual packages because tests are ran outside of them;
• add a bit of overhead because there's now two derivations per package;
• allow actual integration testing, because we would build a new env in which we perform the testing.

I propose we then add a tested attribute to Python packages that corresponds to the second derivation. This is typically the attribute to use for CI as well.