Add unit test and CI

[goneall]

After reviewing the issues caught in the spdx-spec CI runs of PySHACL, we could greatly shorting the error discovery time if we run something similar as a CI in the spec-parser itself.

The quickest implementation would be to basically copy the validation portions of the CI and create a test target with the example file being tested in the spec-parser.

A better implementation would be to have a richer example file with all properties, profiles and fields present (similar to the example files in the SPDX 2 version of spec).

A middle ground would be to translate the SPDX 2 example to SPDX 3.

[goneall]

@zvr I can create a PR for this with the simple approach if you like.