Define 'invert' test_type from VERS test schema

[mjherzog]

@peinjoh has pointed out in:

• Issues in vers-test.schema-0.1.json #68

that the 'invert' test_type is not implemented in the VERS test schema.

The description in the VERS test schema v0.1 file is: "A VERS inversion of a VERS string to a canonical VERS string.", but there is no branch in the test schema for this test type and the description does not explain the purpose of this test type.

We need to define the use case for an 'invert' test type and add a corresponding branch in the test schema or remove it from the schema.
Unsurprisingly there are no examples of an 'invert' test case in vers-spec/tests/

[mjherzog]

@pombredanne Please clarify the use case for the 'invert' test type

[mjherzog]

Per discussion with @pombredanne the use case for the 'invert' test type is in vulnerability reporting where:

• The input VERS constraints are used to define a set of versions that are affected by a vulnerability, and
• You want to invert the VERS constraints to determine the set of versions that are not affected by that vulnerability