Add test suite validation check to CI

[rubensworks]

To reduce the chance of mistakes in test manifests (e.g. #230), we could introduce a test suite validation check.

One option would be to run all tests against a dummy system using a test suite runner (e.g. https://github.com/rubensworks/rdf-test-suite.js).

[afs]

There are two classes of checks:

1. Structural - are tests listed in the entries?
2. Correctness - does the test pass?

For (1), we would need e.g. an "excluded entries" list so a test can be recorded as present but not part of the test suite. (A "disabled" marker wold seem to be a bigger change.) Bonus for checking that the mf:action, mf:result only mention files that exist.

For (2), I am not keen on a CI check that has opinions about whether a test is right or wrong.

Would a dummy system be reporting success on every test? Could that get confusing?

[Tpt]

+1 to Andy, I tend to think we should only have structural tests.

A possible implementation might be to use shacl shapes to validate the manifest structure and some bespoke code to check that only files that exist are mentioned.