Update PURL `test_group` definitions

[mjherzog]

The names/definitions for PURL test groups in purl-test.schema-0.1.json are:

• base: "Test group for base conformance tests for PURL building and parsing."
• advanced: "Test group for advanced tests to support flexible PURL building and parsing."

The proposed new definitions are:

• base: "Tests that are required for conformance with ECMA-427. Base tests are pass/fail."
• advanced: Tests that are more flexible than base tests and are not required for conformance with ECMA-427. Advanced tests may correct errors from test input. **

NB If validation messages are added to the PURL test schema (#692), the output from an advanced test should include a message (or messages) about any corrections in the test output.

[mjherzog]

My conclusion from the discussion to-date is that changing the test_group definitions should suffice - no compelling reason to change the test_group names so long as the definition are clear.