CI tests with external modules don't test PR code

[dtzar]

Describe the bug:
When a PR happens, Azure DevOps will properly pull the effective code when the PR is merged.
However, the Terraform tests will automatically pull the version of the module code which is currently in the master branch in the GitHub repo, effectively not testing any code changes which would impact the test from working.

To Reproduce:
Make a breaking change in one of the core /cluster/azure/aks folders and initiate a PR. The PR will pass since it is simply testing the upstream code.

Expected behavior:
CI tests the effective local module code with the PR using source = "../.." path for the modules instead of the github.com/Microsoft/bedrock reference.

Additional context:
An easy workaround would be the ability to pass in a variable to the source, but this is a problem which has been around since 2015 with Terraform and doesn't appear to be any "feature" coming to make this work. See this issue and related links from there if you care to know more.

Open to suggestions, but likely a solution will have to do a string replacement of all instances of github.com/Microsoft/bedrock/cluster in the environment with ../.. before the tests happen.