Feat: add test stage to terraform template

What does this MR do?

Add the ubiquitous test stage to terraform template.

Related issues

Checklist

Pre-merge

Consider the effect of the changes in this merge request on the following:

• Different pipeline types
• Non-canonical projects:
  • gitlab-foss
  • security
  • dev
  • personal forks
• Pipeline performance

Post-merge

• Consider communicating these changes to the broader team following the communication guideline for pipeline changes