make tofu test generate dynamic mock values

[ollevche]

Resolves #2223

This PR makes tofu test generate null values for dynamic type instead of immediate failure. It doesn't make sense to fail immediately and setting null makes tests more usable in general.

Target Release

1.9.0

Checklist

• I have read the contribution guide.
• I have not used an AI coding assistant to create this PR.
• I have written all code in this PR myself OR I have marked all code I have not written myself (including modified code, e.g. copied from other places and then modified) with a comment indicating where it came from.
• I (and other contributors to this PR) have not looked at the Terraform source code while implementing this PR.

Go checklist

• I have run golangci-lint on my change and receive no errors relevant to my code.
• I have run existing tests to ensure my code doesn't break anything.
• I have added tests for all relevant use cases of my code, and those tests are passing.
• I have only exported functions, variables and structs that should be used from other packages.
• I have added meaningful comments to all exported functions, variables, and structs.

[github-actions]

Reminder for the PR assignee: If this is a user-visible change, please update the changelog as part of the PR.