Maintenance: Use pre-calculated instance names to support new Terraform Test feature

What does this MR do?

This addresses a problem that is present in tests (e.g. in my draft GitLab Dedicated instrumentor MR).

When trying to wrap a module that includes GET in a Terraform test with a mock AWS provider, the plan cannot resolve the for_each loop on aws_ebs_volume.gitlab. This is not a problem when running with a real AWS provider (not clear why it works, must be some optimisation when using tags?).

The attribute d.instance_name now uses a pre-calculated instance name and it can be used in the for_each.

Related issues

Closes: #937 (closed)

Author's checklist

When ready for review, the Author applies the workflowready for review label and mention @gl-quality/get-maintainers:

• Merge request:
  • Corresponding Issue raised and reviewed by the GET maintainers team.
  • Merge Request Title and Description are up-to-date, accurate, and descriptive
  • MR targeting the appropriate branch
  • MR has a green pipeline
  • MR has no new security alerts in the widget from the Secret Detection and IaC Scan (SAST) jobs.
• Code:
  • Check the area changed works as expected. Consider testing it in different environment sizes (1k,3k,10k,etc.).
  • Documentation created/updated in the same MR.
  • If this MR adds an optional configuration - check that all permutations continue to work.
  • For Terraform changes: set up a previous version environment, then run a terraform plan with your new changes and ensure nothing will be destroyed. If anything will be destroyed and this can't be avoided please add a comment to the current MR.
• Create any follow-up issue(s) to support the new feature across other supported cloud providers or advanced configurations. Create 1 issue for each provider/configuration. Contact the Quality Enablement team if unsure.