Create test case identifier

[mjherzog]

As the number and scope of PURL test cases grows we may want to define a unique identifier for each test case.
A common use case is to quickly communicate a set of failing test cases for a PURL implementation tool.

Based on schemas/purl-test.schema-0.1.json, each test case must have a description but there is no requirement that a description is unique within a test file.
It may not be practical to require/generate a unique test case identifier across all PURL test files but it should be practical to require/generate a unique test case identifier within a test file. The simplest approach may be an arbitrary number or other string - it does not need to be sequential within a file because the sequence of the test cases is not meaningful.