Add integration tests for `provisionerd` <-> `terraform`

# Problem

We currently have [matrix tests](https://github.com/coder/terraform-provider-coder/blob/main/.github/workflows/test.yml#L91-L93) that validate that our Terraform provider `coder/terraform-provider-coder` works correctly with various versions of Terraform.

We lack some targeted integration tests in `coder/coder` that validate the compatibility of `provisioner/terraform` against various versions of Terraform.

https://github.com/coder/internal/issues/690 will partially solve this, but it is expected that these will need to focus on a single Terraform version for performance/reliability.

# Suggested Solution

Add integration tests that ensure our Terraform provisioner is able to work with the same set of Terraform versions as tested in `github.com/coder/terraform-provider-coder`. It is not necessary to test the entire workspace creation workflow, but we should at minimum validate with one or more `null_resource`s.