3.3: schema tests for unevaluatedProperties

[ralfhandl]

Many Objects defined in the spec don't allow additional properties.

Test these by taking a minimal instance that passes, inject a property with a very unlikely name and verify that it fails.

• New "fail" test cases for Discriminator Object and External Documentation Object within a Schema Object, defined in meta.yaml
• Table-driven test for Objects defined in schema.yaml with test cases in file minimal-objects.yaml

This brings subschema ("function") test coverage up to 100%.

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• no schema changes are needed for this pull request