Correct `linkml_files.URL_FOR` - add URL resolutiontests

[sneakers-the-rat]

Continues: https://github.com/linkml/linkml-runtime/pull/310 Fix: https://github.com/linkml/linkml/issues/2047

Previously I had fixed some problems with linkml_files but didn't test the URL_FOR function. This is used by the ContextGenerator and RDFGenerator, and causes linkml tests and fail and breaks them.

This PR

• fixes URL_FOR
• switches tests to pytest
• Add requests_cache
• Unskip tests for URL resolution and correctness - these are ~slow-ish, but since we're using them upstream i figure we should test them.
• Skip fail-prone and unused github URL tests - I can't see where these functions are used within the library and they give us 403's constantly, we might want to remove those? i mostly care about the local path functions here, but also i don't think we should have broken stuff lying around if we can help it

edit: confirmed this does fix test_generators/test_rdfgen.py