Provide mechanism to perform integration tests

#### Description

It would be great to be able to debug Packer Plugin source code. Terraform "acceptance" tests are an ideal model; although, I believe Packer team has a better fit usage for the word "acceptance tests" (the Terraform acceptance tests are really integration tests).

Currently its not very easy to debug when writing Packer plugins

#### Use Case(s)

Debugging issues in various Packer plugins

#### Potential References

- https://developer.hashicorp.com/terraform/plugin/sdkv2/testing/acceptance-tests