create integration test framework for coderd provider

[johnstcn]

Create an integration test framework for the provider.

It should do the following:

Given:

• A specific version of Coder (overridable by environment variable)
• An input main.tf
• An expected state of the Coder instance resulting after terraform apply

Then:

• Stand up a Coder instance of that given version using Docker (using in-memory database is fine)
• Set up required overrides to use the local version of the provider,
• Read the input main.tf and execute terraform plan against the Coder instance,
• Validate the expected state against the actual state

Note: "state" here is intentionally left open; it could be derived from the codersdk.Client or actually inspect the tfstate.

It will likely look similar to https://github.com/coder/terraform-provider-coder/blob/main/integration/

[johnstcn]

This is done, with an example test.

https://github.com/coder/terraform-provider-coder/blob/main/integration/integration_test.go