Tests

[ckindermann]

I set up a round-trip test for an ontology that contains example axioms for all OWL constructors.

I added a simple Makefile that

1. builds ldtab.jar,
2. imports & exports the test ontology,
3. uses ROBOT to check whether the input ontology differs from the ontology generated by ldtab export.

I also added a GitHub action that runs make test.

[ckindermann]

We may want the GitHub action to fail when the output of robot diff is not Ontologies are identical.