Add HED 3-specific tests for unique and required tags

[happy5214]

The Hed3Validator class uses its own code path to access the list of tags with the unique and required attributes. Therefore, for code coverage reasons, there should be separate HED 3-specific tests for these two validation checks.

As of the posting of this issue, there are no tags in any released HED 3 stable schema with either attribute. Therefore, this issue serves as a reminder to go back once such a schema is available and add tests using those tags.