Update PURL test schema and documentation

[mjherzog]

We need to rewrite the current version of purl-spec/docs/tests.md because it does not reflect the many changes to the purl-spec tests from:

• Refine Purl type schema #514 and related changes. These changes completely replaced the file: test-suite-data.json with:
  • schema/purl-test.schema-0.1.json
  • tests/spec/specification-test.json
  • tests/spec/types/ - one file per registered PURL type

Beyond those changes, the current content of tests.md is very limited. This issue is to track updates to tests.md with the objective that this file should document:

• The key concepts and terminology for the PURL test schema - currently purl-test.schema-0.1.json, and
• The resolution of the following proposed key changes to that schema:
  • Schema change / purl-test: Add validation test type #692 and
  • Add component-specific spec test files #743

The current draft to update tests.md is:

• PR-nn
• The prior draft Rewrite 2 of tests.md #752 includes extensive discussion sections with questions and proposed changes that need to be resolved to finalize the next version of tests.md.

Some of the key changes in this draft are:

• definition of test_group
• definition of test_type
• addition of a validation test_type with validation messages

The sub-issues cover changes to the PURL test schema and test files to match the current draft.

Caveats

• The scope of this rewrite 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 how to use the test files for a PURL tool.