core: Shim to old p.ApplyFn interface in MockProvider
core: Shim to old p.ApplyFn interface in MockProvider

This is a pretty basic attempt to turn a pair of values into an old-school
diff. It probably won't work correctly for all tests, but hopefully works
well enough that we can just update the remaining tests in-place to use
the new API directly.

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