Add Terraform test step to common-ci-tasks

As part of #26924 (closed) I added a step (MRs 1, 2) to the Terraform template in the pipeline-templates repo to run terraform test. Unfortunately I did this before realising that we're trying to move off pipeline-templates and onto common-ci-tasks.

• Create new task in common-ci-tasks to run terraform test as part of the Terraform template.