Update python unit testing example to also include config

[cep21]

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Issue details

The python unit test at https://github.com/pulumi/examples/tree/master/testing-unit-py does not use or import any configuration. It would be useful to have Pulumi.yaml have a simple config value and show how to do unit testing the recommended way when pulumi uses config (all of our pulumi projects have config)

Affected area/feature

Python