Introduce separate testing scope for reference validation

[liamcervante]

We need to reference outputs and check blocks from the testing scope. This PR introduces a separate function for parsing referencing that can be called from within the testing context. It also updates the existing functions that create hcl.EvalContext objects so they now accept a function to parse references as part of their signature. This way callers can customise the scope by providing their own list of acceptable references via the parse references function.

A follow up PR will actually use the new functionality from the testing scope. I've split those up to keep reviewing things as easy as possible. This means this PR is essentially a refactor, with no behaviour change or new functionality introduced.

This PR:

• Adds GetOutput and GetCheckBlock functions to the lang.Data interface.
• Marks addrs.OutputValue as referenceable
• Adds a new ParseRef function that retrieves a different set of references for within the testing scope
• Updates the lang.Scope struct to dynamically retrieve different references based on the caller, so the testing scope and main context scope can be different.

[jbardin]

I cannot find any cases of this, but something to look out for is code which is meant to exclude things like outputs by checking if the address is referenceable.

[github-actions]

Reminder for the merging maintainer: if this is a user-visible change, please update the changelog on the appropriate release branch.

[github-actions]

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