Introduce an unit testing library

[teneko]

As the scenarios get more and more complex, we need a unit testing library for bootstrapping Terraform more easily. The requirement is to make it consumable by users of PseudoDynamic.Terraform.Plugin.Sdk.

Not only unit testing should be made possible but also integration tests*¹ (see here) and acceptance tests*² (see here).

*¹ Integration tests are those tests where you create a plugin server instance inside the same process as the running test where you can mock parts of the plugin server.

*² Acceptance tests are those tests where you create an process-indepdent plugin server instance and checks the reality after running Terraform against the plugin server.

TODO:

• Fix a bug where Terraform command wrapper produces defragmented output and error messages
• Create the PseudoDynamic.Terraform.Plugin.UnitTesting project with at least a Terraform command wrapper and minimal integration and acceptance test capabilities
• Create an integration for creating HCL files specific to one scenario that gets executed against written integration tests or acceptance tests (maybe CDKTF?)