Rewrite tests.md

[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 tests #692
  • and the organization of test files: Major change / add component-specific test files #743

Some of the key concepts/terminology that need to be clarified are:

• test_group
• test_type
• validation messages
• error messages

The current draft to update tests.md is:

• Rewrite 2 of tests.md #752 which supersedes discussion-draft for tests.md #747
  I did my best to apply the comments from discussion-draft for tests.md #747 to Rewrite 2 of tests.md #752.

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.
• The current 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.