command: Fix TestPlan_outBackend
command: Fix TestPlan_outBackend

We missed fixing this up during the big updates for the new plan/state
models since the failures were being masked by testBackendState being
broken.

This is the same sort of update made to many other tests: add schema to
the mock provider, adjust for the new plan/state types, and make
allowances for the new built-in diffing behavior in core.

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