[SAT] Test Spec: Check for duplicate values under `enum`

[timroes]

We should have a Source acceptance test running on all sources that will ensure that a spec fails, that contains a duplicate enum entry, e.g. as the Kafka Destination does at the moment: https://github.com/airbytehq/airbyte/blob/master/airbyte-integrations/connectors/destination-kafka/src/main/resources/spec.json#L192-L194

[erohmensing]

This seems similar enough to test_oneof_usage to check during spec validation