testing framework: add support for functions in variables and providers

[liamcervante]

This PR adds the base Terraform functions into the testing framework, so that users can access them when defining providers and variables within the testing framework.

Target Release

1.7.0

Draft CHANGELOG entry

ENHANCEMENTS

• terraform test: Terraform functions are now available within variables and provider blocks within test files.

[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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