test: don't panic when resolving references that haven't been evaluated

[liamcervante]

There's a bit of an inconsistency in the way we handle references to objects that haven't been resolved. Resource and local references result in panics, the other references return dynamic values. This PR updates resources and locals so they behave in the same way as other references.

Note, that references to non-existent resources / locals still results in an error as this is checked separately. This isn't normally an issue in regular Terraform as you would have graph cycles if references were circular during regular execution. But, it is possible to assert values against incomplete plans in both terraform test and terraform console and this is where you would see panics. Users can try to check values that weren't included in the plan because of an error or partial targeting, and this would result in panics. After this PR, we'd see unknown values returned for these references which matches the behaviour of modules / outputs / inputs.

Fixes #37409

Target Release

1.13.1

Rollback Plan

• If a change needs to be reverted, we will roll out an update to the code within 7 days.

Changes to Security Controls

Are there any changes to security controls (access controls, encryption, logging) in this pull request? If so, explain.

CHANGELOG entry

• This change is user-facing and I added a changelog entry.
• This change is not user-facing.

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