core: MockProvider shim handling of DiffFn
core: MockProvider shim handling of DiffFn

This should allow many of our existing tests using DiffFn to continue
working with little or no further modification.

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