Pin terraform versions in tests

[lukemassa]

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• I'd be willing to implement this feature (contributing guide)

Describe the user story

@nitrocode recommended in #4462 (comment) that we pin terraform versions in tests. I didn't want to block that particular fix because unit tests were failing, but it seems like a good idea that I'm happy to work on.

Describe the solution you'd like

Terraform providers are pinned in tests such that external updates don't cause tests to start failing.

Describe the drawbacks of your solution

We may fall behind new features, but this is only in unit tests so the risk is low.

Describe alternatives you've considered

We could simply do nothing and live with the occasional test failure. I personally feel strongly about working to make tests idempotent and independent of changes in the "outside world", so I think pinning is the right strategy.