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.

ewbankkit/terraform,youhong316/terraform,tpounds/terraform,youhong316/terraform,pdecat/terraform,rnaveiras/terraform,VladRassokhin/terraform,apparentlymart/terraform,drebes/terraform,drebes/terraform,nlamirault/terraform,youhong316/terraform,apparentlymart/terraform,VladRassokhin/terraform,brandontosch/terraform,brandontosch/terraform,apparentlymart/terraform,iljaskevic/terraform,VladRassokhin/terraform,brandontosch/terraform,mattray/terraform,tpounds/terraform,brandontosch/terraform,luis-silva/terraform,mattray/terraform,gechr/terraform,gechr/terraform,tommynsong/terraform,apparentlymart/terraform,ewbankkit/terraform,ewbankkit/terraform,hashicorp/terraform,pradeepbhadani/terraform,heimweh/terraform,rnaveiras/terraform,pryorda/terraform,brandontosch/terraform,tommynsong/terraform,pdecat/terraform,pryorda/terraform,tpounds/terraform,hashicorp/terraform,luis-silva/terraform,tpounds/terraform,heimweh/terraform,rnaveiras/terraform,luis-silva/terraform,mattray/terraform,heimweh/terraform,nlamirault/terraform,drebes/terraform,iljaskevic/terraform,hashicorp/terraform,pryorda/terraform,pradeepbhadani/terraform,mattray/terraform,luis-silva/terraform,tommynsong/terraform,pdecat/terraform,nlamirault/terraform,heimweh/terraform,youhong316/terraform,pradeepbhadani/terraform,heimweh/terraform,drebes/terraform,iljaskevic/terraform,apparentlymart/terraform,gechr/terraform,ewbankkit/terraform,iljaskevic/terraform,pdecat/terraform,VladRassokhin/terraform