CI: Validate test case manifests

[Ostrzyciel]

• Implement SHACL shapes to validate the manifests
  • Check the basic structure for each test case
  • Check that the test case is attached to the "main" manifest (to avoid issues like the one found in Add test cases for updated spec and general RDF 1.1 conformance #71 )
• Check that the test case files referenced from the manifest exist under the specified paths.
• Maybe also run the test cases? With jelly-jvm and pyjelly, but keep the results of this workflow informative.