Allow users to specify different contract test inputs for each individual test / new resource

I am developing a resource that requires a real, unique activation key as one of its properties.

I have to override inputs because I need to specify real activation keys; a fake key will be rejected. But, I can only specify a single override value, which causes the same key to be reused across tests when running contract tests using `cfn test`.

I can somewhat get around this by running a single contract test at a time and changing the inputs one at a time.

In this case, it would help if I could specify a set of input values so that a different key is chosen for each resource created by the contract tests, and keys aren't repeated.