core: fix TestContext2Apply_outputDiffVars
core: fix TestContext2Apply_outputDiffVars

There were two problems with this test as originally written:
- Its ApplyFn was handling a destroy diff by returning a new object, and
  thus not actually destroying the object in question at all.
- It was treating unknown values in the diff as invalid during apply, but
  these are in fact now expected as a way for the provider to distinguish
  whether an optional+computed attribute is set in config.

With those changes in mind, this test isn't really testing anything
special anymore, but is still a straightforward test of a simple
plan+apply running to completion without error.

hashicorp/terraform-plugin-sdk,hashicorp/terraform-plugin-sdk