Tfexec test helper package for import

As of https://github.com/hashicorp/terraform-exec/pull/51, we run tfexec's e2e tests against Terraform `master`. We should also provide a package for consumers to import, perhaps `tftester`, which allows the tfexec test suite to be used to test the following projects (and others):

- Terraform Core
- terraform-ls
- terraform-plugin-go

Integrating this into the CI for these projects would allow them to test changes against the behaviour of the Terraform CLI before they are merged.