Enforce expected output keys of each stage in the test

[bharathkkb]

TL;DR

With #782 we now use remote state. Stages maybe dependent on the previous states output and if any output key is changed unknowingly we won't will waste build time until a remote data call tries to pull the old key and fails. T

To prevent these kind of failures and enforce stronger guarantees, I propose we add validation at the end of each stage to enforce all expected output keys are present. We could likely use https://github.com/GoogleCloudPlatform/cloud-foundation-toolkit/tree/master/infra/blueprint-test/pkg/golden to create a golden json file with all expected keys and assert at the end of each stage.

Terraform Resources

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Detailed design

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Additional information

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