SkipFunc and more helpful errors for Terraform versions during testing

[kmoe]

ProtoV6ProviderFactories added in https://github.com/hashicorp/terraform-plugin-sdk/pull/761 requires Terraform v0.15.4 or later. Using earlier versions of Terraform will result in an unhelpful error from the Terraform CLI.

Check the Terraform version being used during acc testing and give a helpful error if incompatible provider factory versions are used.

Also provide a built-in SkipFunc for skipping tests based on Terraform versions.

[alexsomesan]

I'd love to see this too. I need to selectively skip acc tests based on TF version.

I our case, this would have to be available via terraform-plugin-test / terraform-plugin-go