Add test traits to harmonize tests.

Problem/Motivation

Writing good tests is not easy. Writing good tests that work for both Config Filter 1.x 2.x and the core API is even more difficult.
But it doesn't have to be!
We can add a test trait that has the same methods in 1.x and 2.x which allows us to assert what is going to be exported and imported. This way the expectations for 1.x and 2.x can be the same and the test could run also when a module switches to the core api.

Steps to reproduce

tbd

Proposed resolution

Add test trait and example test demonsrating its use

Remaining tasks

implement it
open issues on projects using config filter suggesting its use.

User interface changes

none

API changes

new test traits

Data model changes

none