Add integration tests for coder -> terraform-provider-coder

[johnstcn]

Problem

We don't have any integration tests currently in coder/coder targeting version compatibility with coder/terraform-provider-coder. We do have integration tests the other way round (terraform provider -> coder).

Proposed Solution

Adding integration tests in the other direction would allow us to validate that any given version of coder will work correctly with any given version of the terraform provider.

Building blocks for this didn't exist previously but were recently added with coder/coder#15897