minor race issue in mockResourceProvider

The interrupt tests for providers no longer check for the condition
during the diff operation. defer the lock so other test's DiffFns don't
need to be as carefull locking themselves.

terraform/resource_provider_mock.go

@@ -196,13 +196,13 @@ func (p *MockResourceProvider) Diff(
 	info *InstanceInfo,
 	state *InstanceState,
 	desired *ResourceConfig) (*InstanceDiff, error) {
-
 	p.Lock()
+	defer p.Unlock()
+
 	p.DiffCalled = true
 	p.DiffInfo = info
 	p.DiffState = state
 	p.DiffDesired = desired
-	p.Unlock()
 
 	if p.DiffFn != nil {
 		return p.DiffFn(info, state, desired)