Tfexec test helper package for import

[kmoe]

As of #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.