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.

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