Schema testing

[zepatrik]

Related issue

closes #165

Proposed changes

This is a first implementation of our schema "snapshot" tester. It will traverse a success and a failure directory and test the schema against files found in there. It also extracts definitions from that schema to test those individually.

Checklist

• I have read the contributing guidelines
• I have read the security policy
• I confirm that this pull request does not address a security vulnerability. If this pull request addresses a security
  vulnerability, I confirm that I got green light (please contact [email protected]) from the maintainers to push the changes.
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation within the code base (if appropriate)
• I have documented my changes in the developer guide (if appropriate)

Further comments

Will maybe end up in it's own package as a stand-alone cli test tool.

[zepatrik]

Data/snapshots/... could also be generated as we "fixed" circular dependencies by not resolving refs and just checking the src of the ref

[aeneasr]

Looks pretty good to me! Left some suggestions