Test that the language of Frank configs remains backwards-compatible

The Frank!Framework tests (including the Larva tests) hold many Frank configurations. When the language of Frank configs develops, these old configs should stay working. We can add this to the GitHub test of this project, because it does apply itself to the F!F code already (F!F is checked out during test).

There are two ways to do this:
* Feed the checked-out config files to XmlAgainstXsdValidator. Drawback: that tool is not designed thoroughly and does not implement entity references. Extending requires the tool to be unit tested.
* Run the ibis-adapterframework-core JAR headless. Then we use the existing logic of parsing Frank configs (including but not only validation against FrankConfig-compatibility.xsd).

Start by comparing the two proposed approaches: feeding checked-out configs to XmlAgainstXsdValidator or running the ibis-adapterframework-core JAR headless. Review FrankConfig-compatibility.xsd and the existing GitHub test setup, then determine how the checked-out Frank configurations should be exercised. Done means the test verifies that existing configurations remain compatible as the Frank configuration language evolves.

Written by the indexing model from the issue text.