Update names and definitions of PURL `test_group`

[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 names base and advanced are not very clear. The planned new names/definitions are:

• strict: "Tests that are required for conformance with the PURL specification. Strict tests are pass/fail."
• permissive: Tests that are more permissive than base tests. Permissive tests may provide severity messages,
  correct errors from test input, or both.

The primary tasks to implement this change are:

• Update the test_group names and definitions in the PURL test schema - schemas/purl-test.schema-0.1.json.
• Update the test_group name values in all current PURL test files - this should be a one-time mass update.