Update PURL test schema and documentation

[mjherzog]

This issue is to track a set of proposed changes to the PURL test schema, purl-test.schema-0.1.json, and test documentation.

• The file docs/tests.md documents the current state
• The file docs/tests-draft-v0.2.md is a draft to replace tests.md incorporating the changes proposed in this issue (and its sub-issues).

NB The scope of tests.md is to provide an overall description of PURL tests in the context of the PURL test schema. Its scope does not cover how to create a test file or details for how a PURL implementation should use the test file.

A summary of the proposed changes is:

• Update PURL test_group definitions #758 covers editorial changes to the test_group definitions
• Correct spelling of "roundtrip" test_group #801 covers an editorial change
• Schema change / purl-test: Add validation test type #692 adds a new 'validation' test_type and a set of validation messages.

These proposed changes require changes to the PURL test schema and corresponding changes to existing test files. The likely outcome is that we will want to update the schema to v0.2. We should be able to automate the test file updates.
The relevant issues are also attached here as sub-issues.

Major update Feb 5, 2026