Add skipReason field when skipping a test

[PatStLouis]

When skipping tests because an optional feature is not implemented by an implementer, it would be beneficial to have a skipReason field which we could specify the reason for skipping the test (notImplemented in this case). Currently, all skipped tests generate a pending reason.

If we could somehow map a skipReason to and statusMark, this would provide a significant improvement and granularity to report generation:
https://github.com/digitalbazaar/mocha-w3c-interop-reporter/blob/main/lib/statusMarks.js